…ttern

why: The documentation mixed user-facing CLI docs with internal Python
API reference at the same sidebar level, the landing page dumped the
entire README (12 H1 headings), and contributor docs were scattered.
This restructure follows the Python Documentation Skeletons spec where
cli/ is the primary reference surface for a CLI package and the Python
API is explicitly internal.

what:

Structure:
- Move entire docs/api/ to docs/internals/api/ (Python API is internal
  for a CLI package)
- Rename inner api/internals/ to api/_internal/ to avoid path stutter
- Create topics/ directory with workflows, troubleshooting,
  library-vs-cli, and plugins (moved from plugins/)
- Create project/ directory (contributing, code-style, releasing)
- Move developing.md to project/contributing.md
- Fold about.md (stale 2016 content) into topics/index.md as a brief
  tmuxinator/teamocil comparison note
- Delete about.md

New pages:
- cli/exit-codes.md — exit codes for scripting and automation
- cli/recipes.md — copy-pasteable command invocations
- internals/index.md — explicit "not for end users" warning
- internals/architecture.md — CLI dispatch flow diagram
- topics/workflows.md — CI integration, scripting patterns
- topics/troubleshooting.md — common shell/PATH/tmux issues
- topics/library-vs-cli.md — when to use tmuxp CLI vs libtmux,
  concept mapping table, what the CLI can't express
- project/code-style.md — ruff, mypy, NumPy docstrings
- project/releasing.md — git tags, OIDC trusted publishing

Landing page:
- Compose standalone homepage (no README.md includes)
- One-sentence intro, 3+2 grid cards, 3-command install,
  inline YAML example + tmuxp load command, demo GIF

Section indexes:
- cli/index.md: heading "CLI Reference", 2x3 card grid for key
  commands + exit-codes and recipes
- topics/index.md: 2x2 card grid with comparison note
- project/index.md: 2x2 card grid for contributor pages (3 items)
- configuration/index.md: 1x3 card grid for reference subpages

Navigation:
- Sidebar primary: Quickstart, CLI Reference, Workspace files, Topics,
  Internals, Project, Changelog
- Sidebar "More" caption: The Tao of tmux, Migration, Glossary
- 35 redirects for all moved files (every individual api/ file covered)
- README.md URLs updated to new doc structure paths, http → https

Dependencies:
- Add sphinx-design to docs and dev dependency groups
- Annotate all doc dependencies with documentation site URLs

conf.py:
- Add sphinx_design extension
- Add myst_heading_anchors = 4