Remove external site dependencies

[espy]

Overview

This PR fulfills the "external domains" part of #9246 and makes it so the site no longer relies on any CDNs or other external resources that will be blocked by apache.org when we move the site there in the future.

This means:

• removing gravatar and having author avatars in the repo, plus allowing authors without avatars
• removing Google Analytics (currently no replacement planned)
• self-hosting the fonts instead of using Google Fonts
• self-hosting Bootstrap and JQuery
• self-hosting pouchdb.min.js instead of linking to a CDN. This is a bit annoying, since it requires adding the built PouchDB library to the website assets after a version update. The site needs to be updated anyway, but it's important that the new pouchdb.min.js is also added to it. To make this clearer, I've also added a WEBSITE_README.md.
• Fixed date formatting in feed.xml
• Made sure the site can be hosted/staged on GH pages (so in a subdirectory) by prefixing all necessary relative URLs with {site.baseurl}

Testing recommendations

• Build the site with npm run build-site and test it locally, or:
• Go to our staging instance which currently hosts this PR

Related Issues or Pull Requests

#9246

Checklist

• I am not a bot
• This is my own work, I did not use AI, LLM's or similar technology for code or docs generation
• Code is written and works correctly

No tests, types, or website content changes necessary.

[janl]

approved with minor nitpicks