Fix broken internal docs links

[RathodDarshil]

Summary

Fix stale internal docs links and broken anchors found during the broken-link audit.

• update old SDK usage, SKAN, deep-linking, and API routes to current docs paths
• fix stale heading fragments for SDK signup, Meta Commerce Manager, SKAN postbacks, TikTok FAQs, and testing anchors
• remove invalid remarketing audience fragments that pointed to missing headings

Verification

• git diff --check
• rendered local Mintlify internal page and anchor check
• npx mintlify broken-links (still reports existing TikTok %20 image refs, which are outside this PR)