fix(www): drop malformed legal URLs from sitemap_www.xml (#45775)

## Summary
Google Search Console flagged 4 "URL not allowed" errors on
`sitemap_www.xml` — malformed URLs like
`https://supabase.comdata/legal/terms/v1` (missing slash, non-existent
path). The generator was globbing `data/**/*.mdx`, picking up the 4
content-source MDX files under `data/legal/` that are imported by
`pages/terms.tsx` and `pages/enterprise-terms.tsx` but are not
themselves routed. With no path replacement mapping `data/...` to a
route and no leading slash, the URL template concatenated to garbage.
The real `/terms` and `/enterprise-terms` URLs come from the
`pages/*.tsx` glob and are unaffected.

## Changes
- Remove `data/**/*.mdx` glob (and its companion `!data/*.mdx` exclude)
from the sitemap generator. `apps/www/data/` has no routed MDX, only
content sources imported into pages.
- Anchor the `pages` prefix replace: `.replace('pages', '')` →
`.replace(/^pages/, '')`. String-form replace is first-occurrence and
would mangle any future filename containing `pages` as a non-prefix
substring (e.g., `_blog/about-pages.mdx` → `/blog/about-`). No current
files trigger this; defensive hardening.

## Testing
Regenerated the sitemap locally and verified:
- [x] `grep -c "supabase.comdata" public/sitemap_www.xml` → `0` (was 4)
- [x] `<loc>https://supabase.com/terms</loc>` and
`<loc>https://supabase.com/enterprise-terms</loc>` still present
- [x] Every `<loc>` matches
`^<loc>https://supabase\.com(/[a-zA-Z0-9].*)?</loc>$` (no malformed URLs
of any kind)
- [x] Total loc count stable across both commits (regression-free for
the anchor change)

Local count is lower than prod (527 vs 906) because
`.next/server/pages/**` partner/expert/feature HTML globs only resolve
after a full build — runs correctly via `postbuild` on Vercel.

After deploy lands, resubmit `sitemap_www.xml` in Google Search Console
to force a re-crawl (otherwise daily-ish). Expect status to flip from "4
errors" to "Success" and Discovered pages: 906 → 902.

## Linear
- fixes GROWTH-837

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

## Summary by CodeRabbit

## Release Notes

* **Chores**
* Improved sitemap generation to properly index specific content
sections (blog, case studies, customers, events, and alternatives) with
refined route path processing for better search engine discoverability.

[![Review Change
Stack](https://storage.googleapis.com/coderabbit_public_assets/review-stack-in-coderabbit-ui.svg)](https://app.coderabbit.ai/change-stack/supabase/supabase/pull/45775)

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

apps/www/internals/generate-sitemap.mjs

@@ -10,13 +10,11 @@ async function generate() {
     'pages/*.tsx',
     'pages/*.mdx',
     'pages/**/*.tsx',
-    'data/**/*.mdx',
     '_blog/*.mdx',
     '_case-studies/*.mdx',
     '_customers/*.mdx',
     '_events/*.mdx',
     '_alternatives/*.mdx',
-    '!data/*.mdx',
     '!pages/_*.js',
     '!pages/_*.tsx',
     '!pages/api',
@@ -38,7 +36,7 @@ async function generate() {
     .map((page) => {
       const path = page
         .replace('.next/server/pages', '')
-        .replace('pages', '')
+        .replace(/^pages/, '')
         .replace('.html', '')
         // add a `/` for blog posts
         .replace('_blog', `/${blogUrl}`)
@@ -120,7 +118,9 @@ async function generate() {
   const changelogDetailUrls = (() => {
     try {
       const rss = readFileSync('public/changelog-rss.xml', 'utf-8')
-      const matches = [...rss.matchAll(/<link>(https:\/\/supabase\.com\/changelog\/\d+[^<]*)<\/link>/g)]
+      const matches = [
+        ...rss.matchAll(/<link>(https:\/\/supabase\.com\/changelog\/\d+[^<]*)<\/link>/g),
+      ]
       const uniqueUrls = [...new Set(matches.map((match) => match[1]))]
 
       return uniqueUrls.map(