fix(docs): tighten onBrokenLinks to throw and fix surfaced broken links

Previously docusaurus.config.ts had `onBrokenLinks: 'warn'`, so broken
internal links produced advisory warnings during build but didn't gate
merges. Tightening to `throw` surfaces every broken internal route at
build time. Three classes of issue fell out:

1. Stale `/docs/...` and `/docs/6.0.0/...` references in the 6.0.0
   versioned snapshot. The user-facing docs section was renamed
   `docs` → `user-docs` (routeBasePath) at some point after 6.0.0 was
   cut, but the snapshot's links still pointed at the old prefix. The
   live site redirects /docs/* → /user-docs/* at runtime, but
   Docusaurus's onBrokenLinks checker doesn't honor redirects.
   Bulk-rewrote /docs/* → /user-docs/* across the snapshot (and one
   /docs/api → /developer-docs/api).

2. Bare-relative MDX links like `[Label](./mcp)` (no .md/.mdx
   extension). Docusaurus renders an absolute href in SSR HTML, so
   static crawlers see correct links — BUT React Router's `<Link>`
   component on the client side resolves the bare path relative to
   the current URL on click, so when the page URL has a trailing
   slash (e.g. /extensions/overview/), `./mcp` becomes
   /extensions/overview/mcp (404). This is exactly the broken-flow a
   user reported on /developer-docs/extensions/overview/. Added the
   `.md`/`.mdx` extension to all 44 such links across 17 files; this
   makes Docusaurus resolve them to the canonical doc URL at the
   <Link> level, so SPA navigation works regardless of trailing slash.

3. Miscellaneous content fixes:
   - 4 `/configuration/feature-flags` references in 6.0.0 snapshot
     pointed at a page that doesn't exist in that version (the
     dedicated feature-flags page was added later). Repointed to the
     `#feature-flags` anchor inside `configuring-superset.mdx`.
   - 3 references to `superset-core/src/superset_core/rest_api/decorators.py`
     in extensions docs were rendered as relative URLs, resolving to
     /developer-docs/extensions/superset-core/... (404). Converted to
     absolute GitHub URLs.
   - 1 `/storybook/?path=...` link in extensions/components/index.mdx
     pointed at a non-existent route. Repointed to the existing
     `/developer-docs/testing/storybook` page that explains how to
     run Storybook locally.
   - 4 unclosed-paren markdown links in 6.0.0 installation-methods.mdx
     (pre-existing source bugs).

Build now passes with `onBrokenLinks: 'throw'`. Note that
`onBrokenAnchors` is still `'warn'` (default); a separate effort
should tighten that and fix the surviving anchor warnings (currently
~60 instances of `/community#superset-community-calendar`).

docs/versioned_docs/version-6.0.0/faq.mdx

@@ -51,11 +51,11 @@ multiple tables as long as your database account has access to the tables.
 ## How do I create my own visualization?
 
 We recommend reading the instructions in
-[Creating Visualization Plugins](/docs/6.0.0/contributing/howtos#creating-visualization-plugins).
+[Creating Visualization Plugins](/user-docs/6.0.0/contributing/howtos#creating-visualization-plugins).
 
 ## Can I upload and visualize CSV data?
 
-Absolutely! Read the instructions [here](/docs/using-superset/exploring-data) to learn
+Absolutely! Read the instructions [here](/user-docs/using-superset/exploring-data) to learn
 how to enable and use CSV upload.
 
 ## Why are my queries timing out?
@@ -142,7 +142,7 @@ SQLALCHEMY_DATABASE_URI = 'sqlite:////new/location/superset.db?check_same_thread
 ```
 
 You can read more about customizing Superset using the configuration file
-[here](/docs/6.0.0/configuration/configuring-superset).
+[here](/user-docs/6.0.0/configuration/configuring-superset).
 
 ## What if the table schema changed?
 
@@ -157,7 +157,7 @@ table afterwards to configure the Columns tab, check the appropriate boxes and s
 
 To clarify, the database backend is an OLTP database used by Superset to store its internal
 information like your list of users and dashboard definitions. While Superset supports a
-[variety of databases as data _sources_](/docs/6.0.0/configuration/databases#installing-database-drivers),
+[variety of databases as data _sources_](/user-docs/6.0.0/configuration/databases#installing-database-drivers),
 only a few database engines are supported for use as the OLTP backend / metadata store.
 
 Superset is tested using MySQL, PostgreSQL, and SQLite backends. It’s recommended you install
@@ -190,7 +190,7 @@ second etc). Example:
 
 ## Does Superset work with [insert database engine here]?
 
-The [Connecting to Databases section](/docs/6.0.0/configuration/databases) provides the best
+The [Connecting to Databases section](/user-docs/6.0.0/configuration/databases) provides the best
 overview for supported databases. Database engines not listed on that page may work too. We rely on
 the community to contribute to this knowledge base.
 
@@ -226,7 +226,7 @@ are typical in basic SQL:
 ## Does Superset offer a public API?
 
 Yes, a public REST API, and the surface of that API formal is expanding steadily. You can read more about this API and
-interact with it using Swagger [here](/docs/api).
+interact with it using Swagger [here](/developer-docs/api).
 
 Some of the
 original vision for the collection of endpoints under **/api/v1** was originally specified in
@@ -266,7 +266,7 @@ Superset uses [Scarf](https://about.scarf.sh/) by default to collect basic telem
 We use the [Scarf Gateway](https://docs.scarf.sh/gateway/) to sit in front of container registries, the [scarf-js](https://about.scarf.sh/package-sdks) package to track `npm` installations, and a Scarf pixel to gather anonymous analytics on Superset page views.
 Scarf purges PII and provides aggregated statistics. Superset users can easily opt out of analytics in various ways documented [here](https://docs.scarf.sh/gateway/#do-not-track) and [here](https://docs.scarf.sh/package-analytics/#as-a-user-of-a-package-using-scarf-js-how-can-i-opt-out-of-analytics).
 Superset maintainers can also opt out of telemetry data collection by setting the `SCARF_ANALYTICS` environment variable to `false` in the Superset container (or anywhere Superset/webpack are run).
-Additional opt-out instructions for Docker users are available on the [Docker Installation](/docs/6.0.0/installation/docker-compose) page.
+Additional opt-out instructions for Docker users are available on the [Docker Installation](/user-docs/6.0.0/installation/docker-compose) page.
 
 ## Does Superset have an archive panel or trash bin from which a user can recover deleted assets?