minor documentation improvements

I've just gone through the documentation and made some changes to hopefully clear up the documentation as the current state of it is... "jank", as best as I can put it.

Currently, I've only redid most of the "Fresh installation" page. I haven't touched anything else aside from FAQs to add a possible solution to a possible problem.

It's mostly just worded better in some cases to make it seem less confusing and linking to more areas rather than assuming the user already knows where to look for everything. A few changes I did make other than that:

• Added Caddy to the "Configure the web server" section
• Added "Image/file upload still isn’t working!" to "FAQs" (From a personal issue I've had setting up Sharkey)

I plan on doing more, but I think this is good enough to submit for now.