fix(docs): use .mdx (not .md) for generated component-page links

CI caught a follow-on bug from the previous commit. The component
files emitted by generate-superset-components.mjs are `.mdx`, but
my generator patch appended `.md` to the link suffix. Result: the
generated index pages (ui/index.mdx, design-system/index.mdx, and
the build-time-generated extension/index.mdx) reference
`./autocomplete.md`, `./dropdowncontainer.md`, etc. — all targets
that don't exist (the real files are `.mdx`). Docusaurus's
`onBrokenMarkdownLinks: 'throw'` correctly bombs the build.

Worth calling out: this is exactly the failure mode the previous
commit was trying to fix — bare relative links 404'd silently; now
that they go through the file resolver, the resolver catches the
extension mismatch instead. The system is working; my fix was just
wrong on this one detail.

Two corrections:

1. generate-superset-components.mjs now emits `.mdx` suffix (not
   `.md`) on the component links it produces, matching the actual
   page file format. Comment updated to make the intent explicit
   for future maintainers.

2. The two committed auto-generated index files
   (developer_docs/components/ui/index.mdx and
   developer_docs/components/design-system/index.mdx) had 46 + 7 =
   53 `.md` link suffixes from the previous commit. Bulk-corrected
   to `.mdx` to match their actual targets. The third generated
   file (components/extension/index.mdx) is produced at build time
   and not committed; the generator fix above handles it.

Verified with a re-audit script: 92/92 markdown-link targets across
the PR's modified files now resolve to real `.md` / `.mdx` files on
disk. lint-docs-links still passes (it only fails on missing
extensions, not on this category of mismatch — that's `onBroken
MarkdownLinks`'s job, and it WAS doing its job here).

docs/developer_docs/components/design-system/index.mdx

@@ -29,10 +29,10 @@ sidebar_position: 1
 
 ## Components
 
-- [DropdownContainer](./dropdowncontainer.md)
-- [Flex](./flex.md)
-- [Grid](./grid.md)
-- [Layout](./layout.md)
-- [MetadataBar](./metadatabar.md)
-- [Space](./space.md)
-- [Table](./table.md)
+- [DropdownContainer](./dropdowncontainer.mdx)
+- [Flex](./flex.mdx)
+- [Grid](./grid.mdx)
+- [Layout](./layout.mdx)
+- [MetadataBar](./metadatabar.mdx)
+- [Space](./space.mdx)
+- [Table](./table.mdx)

docs/developer_docs/components/ui/index.mdx

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