docs(rust): make it a standalone Rust README (drop Python framing)

Rename rust/PORTING.md -> rust/README.md and rewrite as a Rust-only usage doc:
drop the Python->Rust module map, the parity audit, the Python backward-compat
section, and the "vs Python" deltas. Keep build/run, architecture, the
"How it uses the CocoIndex Rust SDK" walkthrough, CLI commands, configuration,
testing, and a plain limitations/follow-ups list. Update the e2e fixture that
copies the doc as a sample markdown file.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: badmonster0