Sphinx-external-toc

- 👌 IMPROVE: Add `ensure_index_file` event on build completion
- ♻️ REFACTOR: Rename key: `parts` -> `subtrees`
- ♻️ REFACTOR: `sections` -> `items`, and add `format`
- The `format` key adds key-mapping for jupyter-book support.
- ♻️ REFACTOR: API naming: renamed to be more general:
- `DocItem` -> `Document`
- `DocItem.parts` -> `Document.subtrees`
- `TocItem` -> `TocTree`
- `TocItem.sections` -> `TocTree.items`

🐛 FIX: `numbered: true`, this was being equated to `numbered: 1` rather than `numbered: 999` (i.e. infinite depth).

- ♻️ REFACTOR: Move parsing code to separate module
- ✨ NEW: Improve option parsing, add jupyter-book migration

👌 IMPROVE: `toc.yml` validation

🐛 FIX: for nested docnames in sub-folders

Initial alpha release.