Feature: support for MyST markdown input

[offbyone]

This adds support for the Markedly Structured Text format, which is a hybrid of Markdown and some of RST's better features.

https://myst-parser.readthedocs.io/

Pull Request Checklist

Resolves: #issue-number-here

• Ensured tests pass and (if applicable) updated functional test output
• Conformed to code style guidelines by running appropriate linting tools
• Added tests for changed code
• Updated documentation for changed code

[minchinweb]

Extensions like this are often done via Plugin rather than directly in Pelican core.

Also, there already is a MyST Reader plugin that might do what you're after --> https://pypi.org/project/pelican-myst-reader/

[offbyone]

I'm thinking that it might be worthwhile to consider a more capable markdown parser. The extension is great, but I'm more interested in bringing it into core. I'm not attached to this; it's been a fun side experiment, and it's a draft for a reason.

Rather than a back and forth here, though, #3553 is a good place to talk about it.