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)