✨ NEW: Adding - chapter entries to _toc.yml

[choldgraf]

This adds support for - chapter: entries in _toc.yml. It re-works how the TOCtrees are build from _toc.yml. We now allow for two ways to create your sections:

1. Using a flat list of - file: entries
2. Using explicit chapters w/ groups of files with - chapter: entries.

I still need to update the tests to use this new structure, but want to know if folks think this looks satisfactory.

deprecates:

• the header: option in _toc.yml. If you want headers, you should now use - chapter: My title groupings.
• controlling numbered: true directly on files. If you wish to specify that single file / group of files are numbered, do so via - chapter: groupings.

✨ See new docs for this behavior here ✨

closes #783

The following release notes are used in our tests to make sure they work properly!

Release notes

Here's an example of the TOC structure:

- file: intro
  numbered: true

- chapter: Get started
  sections:
  - file: start/overview
  - file: start/build

- chapter: Book pages and types
  sections:
  - file: content/markdown
  - file: content/notebooks

- chapter: Reference and test pages
  sections:
  - file: test_pages/test
    sections:
      - file: test_pages/layout_elements
      - file: test_pages/equations

[codecov]

Codecov Report

Merging #817 into master will increase coverage by 2.11%.
The diff coverage is 97.50%.

@@            Coverage Diff             @@
##           master     #817      +/-   ##
==========================================
+ Coverage   82.51%   84.62%   +2.11%     
==========================================
  Files           8        8              
  Lines         669      657      -12     
==========================================
+ Hits          552      556       +4     
+ Misses        117      101      -16     

Flag	Coverage Δ
#pytests	84.62% <97.50%> (+2.11%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
jupyter_book/toc.py	93.82% <97.50%> (+8.76%)	⬆️

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[choldgraf]

OK I'm gonna merge this one in so that we can have time to try it out on master 👍