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/components/ui/index.mdx

@@ -29,49 +29,49 @@ sidebar_position: 1
 
 ## Components
 
-- [AutoComplete](./autocomplete)
-- [Avatar](./avatar)
-- [Badge](./badge)
-- [Breadcrumb](./breadcrumb)
-- [Button](./button)
-- [ButtonGroup](./buttongroup)
-- [CachedLabel](./cachedlabel)
-- [Card](./card)
-- [Checkbox](./checkbox)
-- [Collapse](./collapse)
-- [DatePicker](./datepicker)
-- [Divider](./divider)
-- [EditableTitle](./editabletitle)
-- [EmptyState](./emptystate)
-- [FaveStar](./favestar)
-- [IconButton](./iconbutton)
-- [Icons](./icons)
-- [IconTooltip](./icontooltip)
-- [InfoTooltip](./infotooltip)
-- [Input](./input)
-- [Label](./label)
-- [List](./list)
-- [ListViewCard](./listviewcard)
-- [Loading](./loading)
-- [Menu](./menu)
-- [Modal](./modal)
-- [ModalTrigger](./modaltrigger)
-- [Popover](./popover)
-- [ProgressBar](./progressbar)
-- [Radio](./radio)
-- [SafeMarkdown](./safemarkdown)
-- [Select](./select)
-- [Skeleton](./skeleton)
-- [Slider](./slider)
-- [Steps](./steps)
-- [Switch](./switch)
-- [TableCollection](./tablecollection)
-- [TableView](./tableview)
-- [Tabs](./tabs)
-- [Timer](./timer)
-- [Tooltip](./tooltip)
-- [Tree](./tree)
-- [TreeSelect](./treeselect)
-- [Typography](./typography)
-- [UnsavedChangesModal](./unsavedchangesmodal)
-- [Upload](./upload)
+- [AutoComplete](./autocomplete.md)
+- [Avatar](./avatar.md)
+- [Badge](./badge.md)
+- [Breadcrumb](./breadcrumb.md)
+- [Button](./button.md)
+- [ButtonGroup](./buttongroup.md)
+- [CachedLabel](./cachedlabel.md)
+- [Card](./card.md)
+- [Checkbox](./checkbox.md)
+- [Collapse](./collapse.md)
+- [DatePicker](./datepicker.md)
+- [Divider](./divider.md)
+- [EditableTitle](./editabletitle.md)
+- [EmptyState](./emptystate.md)
+- [FaveStar](./favestar.md)
+- [IconButton](./iconbutton.md)
+- [Icons](./icons.md)
+- [IconTooltip](./icontooltip.md)
+- [InfoTooltip](./infotooltip.md)
+- [Input](./input.md)
+- [Label](./label.md)
+- [List](./list.md)
+- [ListViewCard](./listviewcard.md)
+- [Loading](./loading.md)
+- [Menu](./menu.md)
+- [Modal](./modal.md)
+- [ModalTrigger](./modaltrigger.md)
+- [Popover](./popover.md)
+- [ProgressBar](./progressbar.md)
+- [Radio](./radio.md)
+- [SafeMarkdown](./safemarkdown.md)
+- [Select](./select.md)
+- [Skeleton](./skeleton.md)
+- [Slider](./slider.md)
+- [Steps](./steps.md)
+- [Switch](./switch.md)
+- [TableCollection](./tablecollection.md)
+- [TableView](./tableview.md)
+- [Tabs](./tabs.md)
+- [Timer](./timer.md)
+- [Tooltip](./tooltip.md)
+- [Tree](./tree.md)
+- [TreeSelect](./treeselect.md)
+- [Typography](./typography.md)
+- [UnsavedChangesModal](./unsavedchangesmodal.md)
+- [Upload](./upload.md)