docs(fix[cli]): rename manual "Usage" heading to avoid argparse Id collision

why: The CLI Reference page has both a manual `## Usage` MyST
heading (showing `g <cmd>` -> underlying VCS invocation
equivalences) and a `.. argparse::` directive block that emits
its own auto-generated "Usage" section (showing the synopsis
line).  Both produced the docutils anchor id `usage` on the same
page, tripping a `CRITICAL: Duplicate ID: "usage" [docutils]`
warning and failing `sphinx-build -W`.
what:
- Rename the manual section from `## Usage` to
  `## Usage examples`.  The new slug `usage-examples` no longer
  collides with argparse's `usage`, and the heading describes
  the actual content better (it's a collection of equivalence
  examples, not a synopsis).
- No content changes; only the heading text.
- With this + the prior ref-link commit, `uv run sphinx-build -W
  -b dirhtml docs docs/_build/html` now passes cleanly (was: 6
  warnings treated as errors).

Author: tony

docs/cli/index.md

@@ -31,7 +31,7 @@ When you run `g`, it:
 
 **Note:** `--version`/`-V` is handled by g itself rather than passed to the VCS.
 
-## Usage
+## Usage examples
 
 ```console
 $ g status