Sphinx-external-toc

🐛 FIX: File extensions in ToC

Ensure files are still matched, if they are provided with file extensions.

- ⬆️ UPDATE: Relax dependency pinning to allow Sphinx v4

- ‼ BREAKING: the CLI command `to-site` is now `to-project`, and `from-site` is now `from-project`

- ♻️ REFACTOR: key `items` -> `entries`
- 👌 IMPROVE: Add `--output` to `migrate` command

- 👌 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