Add a list of all available directives

[choldgraf]

From executablebooks/MyST-Parser#81 @nathancarter brought up a good point that right now it can be confusing to authors exactly what directives they have available. @chrisjsewell mentioned that we might be able to auto-generate a list that Jupyter Book has by default:

I did something like this in https://github.com/chrisjsewell/rst-language-server and have used the result from that to populate the test fixtures in: https://github.com/executablebooks/MyST-Parser/tree/master/tests/test_renderers/fixtures (e.g. sphinx_directives.md, docutils_directives.md), and also to generate the auto-completes in https://marketplace.visualstudio.com/items?itemName=ExecutableBookProject.myst-highlight.

This issue is to track this docs improvement

[chrisjsewell]

This will now be achieved in https://github.com/executablebooks/sphinx-ext-autodoc