Formatting fixes

[fredrik-bakke]

Just a bunch of miscellaneous formatting.

• Fix capitalization of module titles
• Make comment headers into actual headers
• Remove some superfluous whitespace
• Organize VSCode settings
• Add VSCode settings that nest ".agdai" and ".agda~" files under the associated ".lagda.md" file to declutter the file view.
• Fix a breaking bug with generate_main_index when there are duplicate titles
• Add <details> blocks to all files (except foundation-core.universe-levels)
• Remove unused imports

[fredrik-bakke]

I'll spare anyone from having to go through these changes, but I would like to point out one issue that remains.

There is a big bunch of references to theorems and exercises in the library without referencing any book. I am okay with having a section header state

### Theorem 12.3

But then the source should be referenced. Therefore, I've left headers like this as comments for now.

[fredrik-bakke]

I will merge this PR once the check has finished.