Restructure/improve docs

Made several changes on the doc, especially:

• restructuredd/refactored install docs
• Added missing pages to TOC when needed
• Minimal wording/layout/typos improvements
• Added new Uninstall docs
• Fixed #1345 (closed)
• Fixed all warnings in sphinx build process (restructuredText syntax errors, etc.)
• Updated certbot instructions to use snap instead of distro packages, as recommended per certbot themselves.
• Added favicon to docs
• Changed authorship of doc from "Eliot Berriot" to "The Funkwhale Collective"
• Committed changelog fragments