Add Markdown renderer as superfences formatter

[waylan]

Description

If a code block is demonstrating Markdown syntax, if can be assigned the md-render attribute, and both the Markdown source and HTML output will be rendered in a nested set of code blocks. This was inspired by PyMdown's custom formatter. The difference is that both the source and output are rendered from a single block, whereas PyMdown's documentation requires defining the two blocks separately. Examples of what the output looks like can be found throughout PyMdown's documentation.

The Documentation Style Guide was updated for this and various other changes related to the switch to Zensical.

AI Assistance Disclosure

• No AI tools were used in preparing this PR.
• If AI tools were used, I have disclosed which ones, and fully reviewed and verified their output.

Checklist

• This PR follows the contribution guidelines.
• The code follows the Code Style Guide.
• The commit message follows the Commit Message Style Guide.
• I have added or updated relevant docs, including release notes if applicable which follow the [Documentation Style Guide](Documentation Style Guide).
• I have added or updated relevant tests.
• I have not requested, and will not request, an automated AI review for this PR.

[waylan]

[facelessuser]

Looks good!