Add Footnote Syntax

[chrisjsewell]

This PR introduces footnote syntax to close #98. It also partially addresses #113 and #116

The actual Footnote and FootnoteReference markdown tokens are implemented upstream in mistletoe-ebp:

• https://mistletoe-ebp.readthedocs.io/en/latest/api/extension_tokens.html#footnote
• https://mistletoe-ebp.readthedocs.io/en/latest/api/extension_tokens.html#footreference

Important: As the new documentation notes, this syntax currently only supports single line footnote definitions. Multiline footnote definitions will be addressed in a later PR.

For the docutils rendering, In my test I noted that, for rST, docutils/sphinx actually seems to handle these pretty poorly;

• not auto-collecting them into a single section at the bottom of the page,
• not making sure the numbers are ordered, by order of reference
• not discarding unreferenced footnotes

In the sphinx doc, they suggest putting them under a header. But I don't see that being an intuitive approach to footnotes.

Therefore, I have 'fixed' all these issues, as you can see in the test example.

tests/test_sphinx/sourcedirs/footnotes/footnote_rst.rst:

tests/test_sphinx/sourcedirs/footnotes/footnote_md.md:

New user documentation of the syntax is here:
https://272-240151150-gh.circle-artifacts.com/0/html/using/syntax.html#footnotes,
and I also improved other aspects of it; turning the syntax lists into tables: https://272-240151150-gh.circle-artifacts.com/0/html/using/syntax.html#block-tokens (I'm sure @choldgraf will appreciate 😄)

@mmcky @rossbar @najuzilu @akhmerov have a look at the above documentation and see what you think!

[chrisjsewell]

The only thing that I now recall could do with improving, is the number ordering of footnote references that are in directives. The issue here is that

1. For the standard markdown parse; the full document is read, without 'processing' roles/directives. When a new footnote reference is encountered, it is added to a global list, which later provides the numbering order.
2. The roles/directives are now processed in a second parse, and any 'nested parsing' is undertaken. Meaning that any new footnote references encountered here will be added to the end of this global list, rather than in the actual order they occur in the document.

Instead of using this list, I probably need to do a final walk of the docutils syntax tree, and record the referencing order.

[chrisjsewell]

The only thing that I now recall could do with improving, is the number ordering of footnote references that are in directives

Fixed 👍

[chrisjsewell]

rebased for merge in #119