chore(typing): address ty diagnostics in docs/ (and decide notebook policy)

[jxnl]

Docs are currently excluded from ty checks (and notebooks are excluded via **/*.ipynb).

Current status

• uv run ty check docs reports 1 diagnostic:
  • docs/hooks/hide_lines.py:26 error[invalid-assignment] (monkeypatch assignment)

Goal

• Decide whether we want ty coverage for:
  • docs/hooks/*.py only, and/or
  • notebooks (docs/tutorials/*.ipynb) via a different tool/process.

Possible approach

• Fix docs/hooks/hide_lines.py with a targeted cast or ignore for the monkeypatch assignment.
• Keep notebooks excluded (or move notebook validation to a separate docs test that doesn’t run through ty).