To keep the `docs/requirements.txt` available and in-sync,
I have added a pre-commit hook that creates it from the `setup.cfg`

This commit removes all code related to the `_toc.yml`,
which is now implemented in https://github.com/executablebooks/sphinx-external-toc.
This implementation adds a range of new features, including:

- `glob` to specify multiple files with unix-style wildcard
- `toctree` options: `caption`, `hidden`, `maxdepth`, `numbered`, `titlesonly`
- The `toctree` are now added directly to the docutils/sphinx AST
  (rather than appending to the source text),
  which means that its is source format agnostic, i.e. will work for rst, md, ipynb, py, ...
- The `tableofcontents` directive is also handled at the same time as the `toctree` insertion,
  rather than later in a post-transform,
  which avoids having to use any builder specific logic.
- The `jupyter-book toc` CLI is now a plugin of the sphinx-external-toc CLI, and is split into multiple commands
  - The new `jupyter-book toc to-site` command allows a template project to be generated directly from a `toc.yml`

The format of the `_toc.yml` has been changed slightly.
To aide users, the code in `jupyter-book build` will check the format is as expected before calling sphinx,
and point users to the new `jupyter-book toc migrate` command.

This also involved updating `jupyterbook-latex` to `v0.3.0`,
and removes the `nested-lookup` dependency.

Co-authored-by: Chris Sewell <chrisj_sewell@hotmail.com>

Co-authored-by: AakashGC <aakashguptachoudhury@gmail.com>

FIXES: https://github.com/executablebooks/jupyter-book/issues/1049

The `_filename_to_title()` function runs `path.with_suffix("")`.

The suffix is currently stripped in  `_content_path_to_yaml()` and then again in `_filename_to_tile()`. If the filename contains a `.` the second call splits the filename at the point.

Correction is: don't strip suffix in   `_content_path_to_yaml(()` .

Co-authored-by: Chris Holdgraf <choldgraf@berkeley.edu>

Ensure sorting of files in directory

This PR brings improvements to `PDF` builds via `LaTeX` by incorporating the
[jupyterbook-latex](https://github.com/executablebooks/jupyterbook-latex

) extension.

This initial release:

1. harmonises the document structure for `html` and `latex` outputs, by implementing support for `part`/`chapter` structures in the `_toc.yml` for PDF via LaTeX,
2. brings support for the jupyter-book directive `tableofcontents` to PDF via LaTeX, and
3. adds support for commonly used tags on `code-cell` directives (such as `hide-cell`) to PDF via LaTeX

Co-authored-by: mmcky <mamckay@gmail.com>
Co-authored-by: mmcky <mmcky@users.noreply.github.com>

* update version and CHANGELOG

* update CHANGELOG

* add in other commits to CHANGELOG

* fix link and PR ref

* suppressing resolve_any_xref issue

* modified fixture files

referencess.bib -> references.bib

minor typo: your --> you