docs: update documentation to reflect Rust CLI release (#1440)

As promised on https://github.com/openai/codex/discussions/1405, we are
making the first official release of the Rust CLI as v0.2.0. As part of
this move, we are making it available in Homebrew:

https://github.com/Homebrew/homebrew-core/pull/228615

Ultimately, we also plan to continue to make the CLI available in npm,
as well, though brew is a bit nicer in that `brew install` will download
only the binary for your platform whereas an npm module is expected to
contain the binaries for _all_ supported platforms, so it is a bit more
heavyweight.

A big part of this change is updating the root `README.md` to document
the behavior of the Rust CLI, which differs in a number of ways from the
TypeScript CLI. The existing `README.md` is moved to
`codex-cli/README.md` as part of this PR, as it is still applicable to
that folder.

As this is still early days for the Rust CLI, I encourage folks to
provide feedback on the command line flags and configuration options.

.github/workflows/ci.yml

@@ -74,7 +74,12 @@ jobs:
           GH_TOKEN: ${{ github.token }}
         run: pnpm stage-release
 
-      - name: Ensure README.md contains only ASCII and certain Unicode code points
+      - name: Ensure root README.md contains only ASCII and certain Unicode code points
         run: ./scripts/asciicheck.py README.md
-      - name: Check README ToC
+      - name: Check root README ToC
         run: python3 scripts/readme_toc.py README.md
+
+      - name: Ensure codex-cli/README.md contains only ASCII and certain Unicode code points
+        run: ./scripts/asciicheck.py codex-cli/README.md
+      - name: Check codex-cli/README ToC
+        run: python3 scripts/readme_toc.py codex-cli/README.md