Niceties to have in the MkDocs site

This issue compiles a list of nice-to-have things in the MkDocs site:

- [X] A "prettier" button for toggling the hidden source code
- [X] Consistent font size for code blocks (the size currently in use is too big in comparison with the font size of the text)
- [x] More readable Agda colorscheme when using dark mode
- [x] Link from each page to the file in the GH repo (in addition or replacing the GH link on top)
- [ ] Automatically unhide code when jumping to it from the search bar
- [ ] Restrict the search bar functionality to md files (or text e.g.,)
- [ ] Rendering of meta information (typechecking times, GH activity ...)
- [x] Table of contents for individual pages on the rhs
- [ ] "Show more Agda" button should probably only show up on pages where there is more code to show---i.e., where there are hidden code blocks.
- [ ] Add hover-over tool tips for Agda tokens.
- [ ] Improve TOC on lhs of site.