fix(docs): omit category sections from reference sitemap (#51069)

The Docs sitemap lists a `/reference/<sdk>/undefined` URL for every
reference category header, on both latest and versioned paths (for
example `/reference/kotlin/v1/undefined`). `generateReferencePages`
turns every flattened section into a link, and category headers never
carry a slug. Search engines get a 404 for the `api/undefined` entries
and a soft 404 for the SDK ones.

I filtered the sections with the same predicate the reference static
params already use (`type !== 'category' && !!slug` in
`Reference.utils.ts`). I ran the generator locally before and after the
change: the only entries it removes are the `/undefined` ones (about 4%
of the sitemap), and it adds none.

**Note:** `getFlattenedSections` stays unchanged because the crawler
route and the reference pages share it.

## To test

Tested on Vercel preview:
- [x] Fetch `/docs/sitemap.xml` on the Docs preview and search for
`/undefined</loc>`: expect no matches
- [x] In the same file, search for `/docs/reference/javascript/select`
and `/docs/reference/kotlin/v1/select`: expect both still listed

## Linear
- fixes GROWTH-1310


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Documentation**
* Reference pages no longer generate links for category sections or
sections without a slug. Existing link paths and priorities remain
unchanged.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

apps/docs/internals/files/reference-lib.ts

@@ -29,12 +29,14 @@ export async function generateReferencePages() {
         .map(async ({ sdkId, version, libPath, isLatestVersion }) => {
           const flattenedSections = await getFlattenedSections(sdkId, version)
           return (
-            flattenedSections?.map((section) => ({
-              link: isLatestVersion
-                ? `reference/${libPath}/${section.slug}`
-                : `reference/${libPath}/${version}/${section.slug}`,
-              priority: 0.8,
-            })) ?? []
+            flattenedSections
+              ?.filter((section) => section.type !== 'category' && !!section.slug)
+              .map((section) => ({
+                link: isLatestVersion
+                  ? `reference/${libPath}/${section.slug}`
+                  : `reference/${libPath}/${version}/${section.slug}`,
+                priority: 0.8,
+              })) ?? []
           )
         })
     )