fix(docs): document KeyOverlay id/name once, on the properties

[CSSFrancis]

The bug

The docs build has been red on every commit since the keys work landed in v0.7.0 — 322029eb was the last green run. It is not specific to any PR; main itself fails at 78b6819e.

KeyOverlay.id and .name are documented twice: once in the class docstring's Attributes block, and again as bare @property definitions that autoclass :members: in docs/api/keys.rst picks up. Sphinx reports each as a duplicate object description, and docs.yml builds with -W:

<unknown>:1: WARNING: duplicate object description of anyplotlib.keys.KeyOverlay.id, other instance in api/keys, use :no-index: for one of them
<unknown>:1: WARNING: duplicate object description of anyplotlib.keys.KeyOverlay.name, other instance in api/keys, use :no-index: for one of them
build finished with problems, 2 warnings (with warnings treated as errors)

Because Deploy docs is gated on needs: build, the published docs have not updated since v0.7.0 either.

The fix

Move the two descriptions onto the properties and drop the Attributes block.

The warning message itself suggests :no-index:, but that just suppresses one of two copies that would still have to be kept in sync by hand. The properties had no docstrings at all, so putting the text there is where it belonged: one copy, at the definition, and the rendered page is unchanged — both descriptions still appear under id and name in the API reference (verified in the built HTML).

Verification

• Full sphinx-build -b html docs build/html -W --keep-going (the exact CI invocation, with the Pyodide wheel staged first): build succeeded, zero warnings, versus 2 before the change on the same tree.
• test_keys + test_documentation: 68 passed.

Independent of #52; either can merge first.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.77%. Comparing base (78b6819) to head (4cd3c68).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #53   +/-   ##
=======================================
  Coverage   90.77%   90.77%           
=======================================
  Files          40       40           
  Lines        4500     4500           
=======================================
  Hits         4085     4085           
  Misses        415      415           

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