Replace PDFs with markdown-rendered pages

[ScriptRaccoon]

Long proofs were previously provided as PDFs, along with their source files, and treated as static assets. In this PR, TeX files are replaced with Markdown files. PDF files are removed and replaced with Markdown-rendered content pages available under /content/[id].

The set-theoretic foundations page (which was already rendered from Markdown) is now also integrated into this system, and its route has been changed from /foundations to /content/foundations.

Advantages

Advantages of Markdown-rendered content pages over PDF files:

• unified content format, reducing the number of ways content can be added
• consistent styling and navigation with the rest of CatDat
• ability to link to other pages within CatDat
• support for dark mode
• fully responsive layout (PDFs are difficult to read on mobile devices)
• support for defined macros
• no binary files (PDFs) in version control
• no need to manually build and sync PDFs
• improved accessibility (screen reader compatibility, text selection, copy paste)

Screenshots

Dark Mode + Mobile

Light Mode + Desktop

Plan

I plan to also replace lemmas with that system. See also #176 for some context.

[ScriptRaccoon]

@varkor [nevermind, the problem is solved now]