Sphinx-external-toc-strict

- 👌 IMPROVE: validate URL: Ensure value of `url` keys match regex used by Sphinx to identify them
- 🐛 FIX: `external_toc_path` with absolute path

- 👌 IMPROVE: Parsing `MalformedError` messages
- 👌 IMPROVE: jupyter-book migration
- Better conversion validation
- Move `options` key above `parts`/`chapters`/`sections` key

- 👌 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