[chore] Vendor mkdocs fonts; update docs dependencies

[tsmethurst]

Description

If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.

If this is a documentation change, please briefly describe what you've changed and why.

Docs builds have been failing recently with mysterious python errors.

On closer inspection, it seems the errors result from a Google fonts error, whereby Roboto fonts couldn't be downloaded (presumably due to some CDN error).

This PR essentially "vendors" the fonts in question by storing them in the ./docs/.cache folder so that they don't need to be downloaded from the CDN on every build. This should prevent our builds from failing when Google fonts is having a whoopsie.

Also updates other docs dependencies to latest versions.

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• I/we have read the GoToSocial contribution guidelines.