Expand contributing docs, README

[askvortsov1]

• Add a screenshot and information about helping Flarum to the docs homepage
• Remove an irrelevant faq item
• Expand on contributing docs a bit, include "why contribute to Flarum"
• Add "other ways to contribute" docs
• Simplify "guide" activeBaseRegex

[askvortsov1]

I notice some of the links to root pages on domains don't have the ending /. That's not strictly required but technically the canonical URL for a homepage always ends with / even if browsers don't show it.

Not exactly sure what you mean by this, could you mention an example?

I haven't checked how this looks in practice, I see some of the text is repeated on multiple pages but I suppose it makes sense.

Refactored to eliminate this. The separate page is now just for docs/translations contributions, everything else is on the homepage.