[WIP] Create FAQ in documentation

[jolheiser]

It would be nice to have an FAQ section for some recurring issues seen on the forums/discord.

Currently there is the troubleshooting page, however it might be worth considering renaming it to FAQ and having it more prominently displayed on the page. (Either up in the navbar, or higher up on the docs page)

Some of the issues could be as simple as a reference to elsewhere in the docs where it is covered, but it might be nice for a casual user to have a single place to look for common problems.

A WIP list of issues we could address:

1. How to migrate from Gogs/GitHub/etc. to Gitea
2. Not seeing a clone URL or the clone URL being incorrect
3. Custom Templates not loading or working incorrectly
4. Several settings combinations that may be confusing or hard to understand how to configure properly
   a. Active user vs login prohibited user
   b. Setting up logging (could simply point to the wonderful page @zeripath is PRing)
5. Adjusting your server for public/private use
   a. How to prevent spammers
   b. Only allow/block certain email domains
   c. Issue only users
6. How to add/use custom themes
7. SSHD vs built-in SSH
8. Why is Gitea running slow? (avatars are usually the go-to culprit)
9. Translation is incorrect/how to add more translations

[jolheiser]

Technically the first one has a page all to itself currently, which could either be moved to the FAQ or just referenced.
https://docs.gitea.io/en-us/upgrade-from-gogs/

However, it might also be nice to add a reference to #4286 as well.