Fix the jEdit install script + documentation

[tkw1536]

I just tried to install the MMT plugin following the instructions at https://svn.kwarc.info/repos/MMT/doc/html/index.html?install.html
I encountered several problems most of which should be fixable:

mkdir content

so that you can later check out content repositories (e.g., the ones in the OAF) into the content folder.

lmh / MathHub should be mentioned here (instead of just OAF).
2) You tell people to run:

svn co https://svn.kwarc.info/repos/MMT system

and a few lines later you say to run the script

MMT/setup.sh

this is inconsistent because we only created a "system" folder. It should also be mentioned that windows users should run the batch script instead.

3. I could not get the automated setup script to run (even when trying the manual installation). It kept failing with:

Error: Could not find or load main class info.kwarc.mmt.jeditsetup.Setup

Running sbt deploy in an attempt to rebuild did not help.

4. After installing manually (via the instructions at https://svn.kwarc.info/repos/MMT/doc/html/index.html?install.html ) I got the plugin itself to run.
   (As a side note: There was only one jar in plugin/jars. Is that intended?)
   After checking out the examples repository and opening a file I got several unbound token errors: