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/developer_docs/extensions/quick-start.md

@@ -168,7 +168,7 @@ class HelloWorldAPI(RestApi):
 
 **Key points:**
 
-- Uses [`@api`](superset-core/src/superset_core/rest_api/decorators.py) decorator with automatic context detection
+- Uses [`@api`](https://github.com/apache/superset/blob/master/superset-core/src/superset_core/rest_api/decorators.py) decorator with automatic context detection
 - Extends `RestApi` from `superset_core.rest_api.api`
 - Uses Flask-AppBuilder decorators (`@expose`, `@protect`, `@safe`)
 - Returns responses using `self.response(status_code, result=data)`
@@ -184,7 +184,7 @@ Replace the generated print statement with API import to trigger registration:
 from .api import HelloWorldAPI  # noqa: F401
 ```
 
-The [`@api`](superset-core/src/superset_core/rest_api/decorators.py) decorator automatically detects extension context and registers your API with proper namespacing.
+The [`@api`](https://github.com/apache/superset/blob/master/superset-core/src/superset_core/rest_api/decorators.py) decorator automatically detects extension context and registers your API with proper namespacing.
 
 ## Step 5: Create Frontend Component
 
@@ -496,7 +496,7 @@ Superset will extract and validate the extension metadata, load the assets, regi
 Here's what happens when your extension loads:
 
 1. **Superset starts**: Reads `manifest.json` from the `.supx` bundle and loads the backend entrypoint
-2. **Backend registration**: `entrypoint.py` imports your API class, triggering the [`@api`](superset-core/src/superset_core/rest_api/decorators.py) decorator to register it automatically
+2. **Backend registration**: `entrypoint.py` imports your API class, triggering the [`@api`](https://github.com/apache/superset/blob/master/superset-core/src/superset_core/rest_api/decorators.py) decorator to register it automatically
 3. **Frontend loads**: When SQL Lab opens, Superset fetches the remote entry file
 4. **Module Federation**: Webpack loads your extension module and resolves `@apache-superset/core` to `window.superset`
 5. **Registration**: The module executes at load time, calling `views.registerView` to register your panel
@@ -509,9 +509,9 @@ Here's what happens when your extension loads:
 
 Now that you have a working extension, explore:
 
-- **[Development](./development)** - Project structure, APIs, and development workflow
-- **[Contribution Types](./contribution-types)** - Other contribution points beyond panels
-- **[Deployment](./deployment)** - Packaging and deploying your extension
-- **[Security](./security)** - Security best practices for extensions
+- **[Development](./development.md)** - Project structure, APIs, and development workflow
+- **[Contribution Types](./contribution-types.md)** - Other contribution points beyond panels
+- **[Deployment](./deployment.md)** - Packaging and deploying your extension
+- **[Security](./security.md)** - Security best practices for extensions
 
 For a complete real-world example, examine the query insights extension in the Superset codebase.