The figures of the overview.pdf file look rather odd although they
used to look better when they were created four years ago. Apparently,
the dia command line tool which creates the pdf from the .dia source
file has changed and now messes up the lines which connect the parts
of the figure.
This commit re-implements four out of the five figures of the overview
file as ascii art in manual.md, removes overview.dia, and modifies
the documentation web page to no longer refer to the dia version.
The text of the overview file has been merged with the manual and
got improved a bit.