docs(gix): document release setup and native conversion gaps

Add installation, validation and coverage guidance for the official
`GixPython==0.1.0` release, using existing Python interpreters and separate
CLI/native environments. Record GixPython/Gitoxide follow-ups with evidence,
reproduction details and the API or behavior needed upstream.

Document the Gix-only boundary: adapters may format native results and select
fallback, but cannot independently implement missing Git semantics. Keep
reference-format queries on Git (`GIX-1`), reflog cleanup on Git (`GIX-10`),
and command-specific committer resolution on Git (`GIX-21`). Native counters
must not label Python substitutes as Gix operations.

Preserve the earlier published-wheel and local-artifact validation records,
including the historical 757.85-second full suite and the future state/fixture
optimization analysis, with their original revisions and scope. These are
historical observations rather than validation of later rewritten trees.

Validation for this documentation correction: checked the capability table
against the amended adapters and passed `git diff --check`. The corresponding
code changes already passed their focused Gix tests followed by CLI tests.
This documentation change reduces CLI calls by zero.

Author: codex

README.md

@@ -17,7 +17,10 @@ probably the skills to scratch that itch of mine: implement `git` in a way that
 If you like the idea and want to learn more, please head over to [gitoxide](https://github.com/Byron/gitoxide), an
 implementation of 'git' in [Rust](https://www.rust-lang.org).
 
-*(Please note that `gitoxide` is not currently available for use in Python, and that Rust is required.)*
+The experimental `GitPython[gix]` extra uses the local GixPython bindings for
+supported operations, with automatic Git CLI fallback. See the
+[local installation, test commands, and conversion ledger](doc/gix-backend.md).
+GixPython is not yet available on PyPI.
 
 ## GitPython