User Guide needs updates

[GitMensch]

The user guide can be created with doing make in the doc directory.

It compiles manext from Mark Hessling which then extracts documentation from the PDCurses sources and some additional parts from the X11 port.
In the end all docs are merged to one final md file which is the User Guide.

Things to check / handle:

• ask Mark Hessling if there's a newer version for manext.c (I've send him a note already, will resend it next week to a different mailing address if needed)
• integrate the SDL2 docs to the Makefile and remove the links during this conversion (I'll do this)
• maybe: add a Win32a.md and/or Win32.md for things that only need to be considered in these ports (@Bill-Gray your part???)
• in any case: check if there are some parts missing, at least PDC_version_info should be added (@Bill-Gray your part?!?) note: most of this seems to be done directly in the pdcurses/*.c sources as this is the root where the information is extracted from
• check to add a TOC, maybe with a program (invoked either during make or once + copied in or directly)
• maybe: add the files for converting the markup file into a PDF (most of them are auto-generated by sphinx-quickstart and tweaked afterwards [currently only a little bit]), maybe even add an optional make target to produce the files (both latex via sphinx and PDF via pdflatex)