Add documentation for public API

[lehnerpat]

There currently exists basically no documentation of the API that devcmd exposes.

We should write some basic documentation of the publicly exposed functionality, at least in the code.

If possible, we should also generate easily-viewable pages from this code doc (e.g. markdown files or so), though we might split this into a separate issue.

[lieberlois]

#27 Adds documentation for the most important parts of the ProcessExecutor class