Document AutoDoc with AutoDoc and use the AutoDoc manual as a test

[gwhitney]

Although the main functions are documented with AutoDoc markup, Chapters 1 and 2 of the manual are currently straight XML. Converting one or both to AutoDoc would provide a showcase of the package's functionality. Then making a test out of generating the AutoDoc manual would provide pretty good coverage of AutoDoc's functionality. Finally, the documentation could then move alongside the code it documents, helping to ensure consistency between code and documentation.

I have proposed code that, among other changes, replaces Comments.xml with Comments.autodoc, and moves the individual command documentation into gap/Parser.gi next to the implementations of the commands. It also adds generating the AutoDoc manual to the test harness started in pull request #166. I will post it soon.