Restructure Docs

This MR restructures the documentation such that it is ordered in a more natural and accessible way.

Summary of changes:

• Restructures the table of contents
• Adds new pages: "about the utopia project", "should i use utopia" and "features"
• Re-categorises several pages, including the tutorials
• Rewrites the "Dev workflow" and "model requirements" pages
• Fixes typos, broken links, misc. errors
• ...

Can this MR be accepted?

• Restructuring finished
• Checked that links still work and docs render correctly
• Pipeline passing without warnings
• Approved by @tgaskin
• Approved by @blsqr
• ⚠ Not squashing (b/c it's a multi-author MR) — rebasing is a big big mess... ok?

Related issues

#259, #253 (closed)