fix: sitemap URLs not XML-encoded and no sitemap index for large sites

[rohilsurana]

Problem

`src/server/routes/sitemap.xml.ts` has two issues:

1. URLs not encoded — slugs with special characters (`&`, `<`, spaces) break XML validity:
   ```xml
   https://docs.example.com/guides/Q&A
   ```

2. No sitemap index — generates a single monolithic sitemap. Google's limit is 50MB / 50,000 URLs. Large doc sites with many API endpoints could exceed this.

Suggested Fix

• XML-encode all URL values in the sitemap
• For sites exceeding 10k URLs, generate a sitemap index pointing to paginated sitemaps