Fix the 5 broken internal links reported in broken-links.txt

Follow-up to #3672.

broken-links.txt at the repo root currently reports 5 broken internal links across 4 files, all pointing to pages that do not exist:

• capabilities/full_text_search/advanced/debug_search_performance.mdx line 98: /resources/self_hosting/sharding
• capabilities/indexing/how_to/document_relations.mdx line 142: /resources/self_hosting/sharding
• capabilities/indexing/how_to/document_relations.mdx line 148: /reference/api/settings/get-foreign-keys
• resources/help/experimental_features_overview.mdx line 69: /capabilities/personalization/getting_started
• resources/self_hosting/enterprise_edition.mdx line 14: /resources/self_hosting/sharding

Beyond the reader-facing dead ends, broken internal links slow down crawler re-mapping of the site after the IA restructures, which matters while we are trying to recover the docs' search positions (see #3672).

Fix is either creating the missing target pages (the sharding page is referenced from 3 places, so it probably should exist) or repointing the links to existing pages.

Start with broken-links.txt and inspect the five links in the four listed .mdx files, then check whether existing documentation can replace the missing targets or whether new pages are needed. Re-run the broken-link check and confirm all five internal links resolve without reader-facing dead ends.