Draft: #1078 Create a ToolDef user manual

I wrote a lot of text for the user-manual in the past two weeks, maybe it's time to see where it is, and what is missing. A detailed review is probably too early, although some pointers would be appreciated.

Note that as I don't know the ToolDef language, I did tests to understand how it works and extended in that direction with educated guesses. It's possible that I missed some behavior or extended in a wrong direction. If you find such a problem, please tell.

I also didn't really consider the relation with the reference manual. I do point at the tool libraries that it describes though.

I am not quite sure what to expect now, probably I need to get some distance first. It seems likely that it will not be finished this year, but we'll see.

Closes #1078

EDIT: Fixed typo.