docs: explain PageIndex runtime patches

[gcharang]

Summary

• explain why the pinned upstream PageIndex runtime requires local patches
• document every patch area, observed corpus failures, artifact identity, and the patch-versus-fork decision
• describe provider-free verification, maintenance rules, and explicit scope boundaries
• update the documentation index with the rationale-focused title

Verification

• python3 scripts/docs_check.py
• git diff --check
• source-level audit against the exact patched runtime in vectorless-rag-api:test