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 interpreters and separate CLI/native
environments. Record upstream requirements with concrete native/Git evidence.

Distinguish available APIs from remaining compatibility gaps. Native config
identifies reference storage and reftable `HEAD` reads report unsupported
storage, but complete repository-extension validation still requires Git.
Gix already exposes linked worktrees of bare main repositories; its documented
`is_bare()` is a configuration flag, not absence of a worktree. Record the
actual dangling-`commondir` and flattened-error issues instead.

Document the missing cheap independent repository clone for per-command
native identity overrides. Ref edits already accept explicit `LogChange`
messages; the missing operation is native Git-compatible whitespace cleanup,
alongside separate HEAD/reflog transaction differences. Keep those fallbacks
and the prohibition on independent Python implementations of Git semantics.

Preserve historical wheel validation, full-suite measurements and future
fixture/state optimization notes with their original revisions and scope.
They are historical observations, not validation of later rewritten trees.

Validation: reviewed the supplied Git/GixPython sources and direct official
GixPython 0.1.0 probes, checked the ledger against the passing native/CLI
regressions, and ran `git diff --check`. This documentation amendment changes
CLI launches by zero.

Correct the README introduction to name the released GixPython 0.2.0
distribution and explicit backend selection. Explain that fixture testing
requires GixPython and Python 3.11+ even for the CLI backend, and link the
setup and compatibility documentation. Whitespace checks passed.

Align the original setup guide with GixPython 0.2.0 and explicit backend
selection in both prepared environments. Preserve historical compatibility
evidence separately from current installation instructions. Whitespace
checks passed; the completed documentation build passed with warnings fatal.

Author: codex

README.md

@@ -17,7 +17,11 @@ 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 GixPython 0.2.0 from PyPI for
+supported operations, with Git CLI fallback. Set `GIT_PYTHON_BACKEND=cli` or
+`gix` to select the backend explicitly. Tests use `gix.testtools` for scripted
+fixtures with either backend and require Python 3.11+. See the
+[installation, test commands, and conversion ledger](doc/gix-backend.md).
 
 ## GitPython