Convert category_archive.html to Markdown

[perlun]

The way it's done now - containing a bunch of .html content specifically for the "programming" category is a bit clumsy.

For reference: https://github.com/perlun/perlun.eu.org/blob/master/_layouts/category_archive.html

We could utilize the approach suggested in jekyll/jekyll#6166 (comment), or try to find a cleaner way to do it.