fix(docs): finish bare-relative link conversion + add lint guardrail

Copilot flagged two stragglers on editors.md where the previous
file-by-file conversion stopped halfway. Sweeping for the same
pattern across the active content tree found 76 bare relative
internal links total — 14 in this PR's already-modified files
(Copilot's two plus twelve more) and 62 in unchanged files.

Why the build doesn't catch this
─────────────────────────────────
`onBrokenLinks: 'throw'` (set in this PR) only validates *file-based*
markdown references — links whose URL ends in `.md` / `.mdx`. Those
go through Docusaurus's file resolver, which can prove the target
exists. Bare relative URL paths like `[Foo](../foo)` skip that
resolver entirely; Docusaurus emits them as raw hrefs. The browser
then resolves them against the *current* page URL, and for
trailing-slash routes that almost always lands in the wrong
directory. Page navigates client-side and 404s. The linkinator job
in CI *can* catch these, but it's `continue-on-error: true` so
findings are advisory.

What this commit does
──────────────────────
1. Fix all 76 bare relative internal links across the active docs
   tree by appending `.md` to each one (preserving anchors / query
   strings). All 76 targets resolved to real files; no link
   targets changed, only the form of the reference.

2. Fix the component-page generator. 54 of the 76 bare links lived
   in two auto-generated index files (`components/ui/index.mdx`
   and `components/design-system/index.mdx`). The next regeneration
   would have undone the manual fixes without this. The two
   emission sites in `generate-superset-components.mjs` now emit
   `.md`-suffixed links; comment at the call site explains why.

3. Add `docs/scripts/lint-docs-links.mjs` — fast source-level
   linter that scans `.md`/`.mdx` files under the active content
   trees (skipping `versioned_docs/` snapshots) and fails if it
   finds any markdown link whose URL starts with `./` or `../` and
   does not end in `.md`/`.mdx`. Excludes asset paths (.png,
   .json, etc.) and ignores fenced code blocks. Wired up as
   `yarn lint:docs-links`.

4. Add a `Lint docs links` step to `superset-docs-verify.yml`,
   running before the build step so PRs that introduce the pattern
   fail in seconds rather than at build-time / not at all. Blocking,
   not advisory — exactly the gap linkinator's `continue-on-error`
   leaves open.

Verified
────────
- `yarn lint:docs-links` exits 0 on the cleaned tree
- Re-introducing one bare link makes the linter report the exact
  file:line with the offending URL, exit code 1
- All 76 originally-flagged targets resolved to real `.md` / `.mdx`
  files; only the form of the reference changed

docs/developer_docs/contributing/guidelines.md

@@ -77,7 +77,7 @@ Finally, never submit a PR that will put master branch in broken state. If the P
     in `requirements.txt` pinned to a specific version which ensures that the application
     build is deterministic.
   - For TypeScript/JavaScript, include new libraries in `package.json`
-- **Tests:** The pull request should include tests, either as doctests, unit tests, or both. Make sure to resolve all errors and test failures. See [Testing](./howtos#testing) for how to run tests.
+- **Tests:** The pull request should include tests, either as doctests, unit tests, or both. Make sure to resolve all errors and test failures. See [Testing](./howtos.md#testing) for how to run tests.
 - **Documentation:** If the pull request adds functionality, the docs should be updated as part of the same PR.
 - **CI:** Reviewers will not review the code until all CI tests are passed. Sometimes there can be flaky tests. You can close and open PR to re-run CI test. Please report if the issue persists. After the CI fix has been deployed to `master`, please rebase your PR.
 - **Code coverage:** Please ensure that code coverage does not decrease.