docs: add API reference and usage guide, fix RTD Python version

[bluetoothbot]

What: Replace the docs skeleton with a real usage guide and an autodoc-based API reference, and fix the Read the Docs Python version.

Why: docs/source/usage.md was a TODO: Document usage stub and there was no API reference (issue #284). Separately, .readthedocs.yml pinned Python 3.9 while the package requires >=3.11 (pyproject.toml), so pip install . — and therefore the whole RTD build — would fail.

How:

• .readthedocs.yml: ubuntu-22.04 + Python 3.12 (compatible with ^3.11).
• conf.py: enable autodoc / autosummary / napoleon / viewcode / intersphinx; pull the version from the package.
• api.md (new): autodoc the public surface — XiaomiBluetoothDeviceData, EncryptionScheme, the cloud bindkey-fetch classes/exceptions, SLEEPY_DEVICE_MODELS. Generated from code, so it won't drift.
• usage.md: real walkthrough — passive parsing, encrypted devices/bindkeys, cloud bindkey fetch, optional active polling.
• installation.md: fix a stray deezer-python PyPI link.

Testing: pip install .[docs] + sphinx-build -b html builds cleanly under Python 3.12 with every documented class/method rendered; isort/black/flake8/codespell pass.

Closes #284.

---

Quality Report

Changes: 6 files changed, 223 insertions(+), 10 deletions(-)

Code scan: 3 issue(s) found

• docs/source/usage.md:30 — debug print statement
• docs/source/usage.md:34 — debug print statement
• docs/source/usage.md:38 — debug print statement

Tests: failed (FAILED)

Branch hygiene: clean

Generated by Kōan

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.00%. Comparing base (2a60814) to head (112b519).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #317   +/-   ##
=======================================
  Coverage   54.00%   54.00%           
=======================================
  Files           7        7           
  Lines        1387     1387           
  Branches      271      271           
=======================================
  Hits          749      749           
  Misses        520      520           
  Partials      118      118           

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