docs: add PrivateLink documentation to manual/core/private_link/

Adds PrivateLink implementation notes to the project's standard
documentation tree (manual/core/private_link/README.md), following the
same structure as other core-driver feature docs. Content covers:
- The PrivateLink problem and abstract routing-table model
- Architecture overview and implementation deep dives
- Shard-awareness through NLBs using Proxy Protocol v2 (PP2)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nikagra