Migrate to Zola

[senekor]

Benefits:

• better maintainability
• link checking
• auto-reloading development server

Downsides:

• It might be necessary to duplicate the date of the post in the front matter (in date and path), because zola doesn't support template strings for the path or something similar. (Maybe we can only have path in the front matter and extract the date from it in our scripts?)

I think we should definitely do #1511 first, for maximum confidence in the transition. There is a good chance Zola will do some things differently. Ideally, we would incrementally change the output of our custom generator, until it matches the output of zola.

[senekor]

At this point, I see two big blockers for making the migration a no-op for the snapshot tests:

• Zola calls our URLs "ugly" and doesn't support them: Support ugly urls getzola/zola#840. We can preserve support for old links with redirects, but the directory structure will be different.
• The generated heading ids for internal links are different than before. I suspect this is because we use comrak and Zola uses pulldown-cmark. I might try to migrate the custom site generator to pulldown-cmark to reduce the final diff, but I also might decide it's not worth it.

I'm thinking I could create two throwaway commits containing the normally-ignored public/ directory. Then the diff can easily be viewed in GitHub's UI, making review of the final migration step more convenient.