Categories need to display their children posts

[setr]

1. Majority of a post is a jinaj2 macro, with post.jinja2 simply embedding it https://tera.netlify.com/docs/templates/#macros
2. Categories imports a post macro for every child that is not a category. It feeds the post struct as input.
3. CSS is done with a nested grid: https://gridbyexample.com/examples/example21/
4. We can serialize structs for tera templates by implementing serde_json's Serialize: https://tera.netlify.com/docs/usage/#usage