Use mdformat for markdown formatting

[jtraglia]

I was reviewing old PRs and saw this:

• Run mdformat #2291 (from March 30th 2021)

After reviewing mdformat I realized this is exactly what we've been looking for! It fixes these issues:

• Enforce some form of markdown style #4171
• Investigate python doctoc replacement #4180

For this, I used the following extensions for GitHub flavored markdown and for table of contents:

• https://pypi.org/project/mdformat-gfm/
• https://pypi.org/project/mdformat_gfm_alerts/
• https://pypi.org/project/mdformat-toc/

The benefits of mdformat...

1. It's so much faster than doctoc (1.2 seconds vs 10.7 seconds, ~9x faster).
2. We don't need to install Node.js for an otherwise pure-python project.
3. It catches formatting problems (eg the missing code block backticks in eip7732 specs).
4. It automatically formats all of our tables.
5. It uses consistent list item markers.
6. It deletes trailing whitespace.
7. It deletes unnecessary double spaces.
8. And more...

I had to make one notable non-formatting change to get this to work:

• Replace [0]: # (eth2spec: skip) with <!-- eth2spec: skip -->

The formatter was removing these footnotes because they unused, makes sense. My solution was to update this to use an HTML comment instead. The change in the pyspec generator was easy and arguably better than before.