DOCS: Split up user guide

• I split up the user guide into separate sections to make it more clear/orderly
• I added a general overview page for the FE from where it links to the other sections that give more detailed information
• I updated outdated information and added new information where I felt was required
• I fixed broken links in other parts of the docs as well

A similar thing could be done for the admin and contributor guide (maybe in a later mr), but I like to get feedback on the userguide first

You can see how the docs look with this MR at https://pleroma-docs-example.ilja.space/ (Choose "Frontend" in the upper tab)