pw_ghish: Reorganize documentation by subcommand and add PR tutorial

Restructure the pw_ghish documentation around its four primary command
families and add an end-to-end PR workflow tutorial:

- Split the monolithic cli.rst into dedicated pages for Gerrit code
  review (pr.rst / gh pr) and LUCI Buildbucket CI (run.rst / gh run).
- Add a new 'Life of a PR' guide (life_of_a_pr.rst) covering the full
  Gerrit + LUCI lifecycle and disambiguating --cq, --auto, and pr merge.
- Consolidate flag_compatibility.rst into cli_comparison.rst and add
  subcommand-specific GitHub CLI comparison sections across pr, run,
  issue, and worktree pages.
- Rename worktrees.rst to worktree.rst (gh wt) and clarify experimental
  status across the module landing page, worktree guide, and roadmap.
- Standardize page titles to compact sentence-cased sidebar headings.

Change-Id: Id5b92e3375ead00908b9b1e7e4ecb6c5d4a1c517
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/484109