Rename Docs to "Learn Matrix" and add it to the Try Matrix page

[MTRNord]

This is a result of a discussion with @HarHarLinks .

The icon illustration is the Gnome UI Icon "document-text" which is CC Zero licensed and was modified to fit the same corner style like the other illustrations. However I would prefer a proper illustration here instead.

[HarHarLinks]

I was comparing the m.org website's categories to where other projects might have "Learn more" and "Get started" sections. To me, "Docs" sounds too dry and not interesting to actually read, but more technical - when actually we want it to be end user facing docs that go a little beyond "getting started" (i.e. jumping in head first to try it out).

We should also clean the section's contents itself up. I find it does not fit the way the rest of the website is designed and the mix of "Matrix for chat" and "Spec guide for implementing authenticated media in the federation API" don't at all fit together.