Improve docs for setting up MMT

[ComFreek]

The setup docs page currently is insufficient for MMT developers.

• [x] Document you still need to install SBT*
• [x] Document you need the Scala plugin within IntelliJ.
• [x] Make a clear distinction for developers and users (perhaps different pages or prefixes of sections?)
• [x] Perhaps spare one sentence that we use IntelliJ as devs for 2 purposes (actual dev + content writing) and they not need be the same IntelliJ installation actually.

Original issue from here: https://fsi.cs.fau.de/forum/post/162634

• [x] Check that everything from the original issue has been incorporated in some way.
• [x] Do endusers (only using the IntelliJ-MMT plugin + mmt.jar) need SBT? If not, how does MMT compile Scala files in archives? edit: no they do not, MMT comes bundled with the Scala compiler, which is invoked via the host system's Java installation.

*) Even though IntelliJ ships with its own sbt binary, you should install that system-wide. IntelliJ's sbt is a) hidden under some installation path and b) works sometimes differently (or hangs), at least in my experience.

[ComFreek]

I overhauled the setting up docs some weeks ago and hopefully made everything clearer.