Shorten AUTODOC_XML_HEADER, remove trailing whitespace in it
Improve Create{Main,Title}Page
CreateTitlePage: Introduce OutWithTag() to improve output

Readability (= "nice looks") of the generated XML seems important for
marketing reasons if we want to encourage use of AutoDoc for creating
documentation.
CreateTitlePage: also show maintainers

Perhaps this should be configurable, or perhaps maintainers should be
marked as such, but they definitely should be mentioned.
Create{Title,Main}Page: Some TODO comments
misc: reduce some minor code duplication
Moved Install doc entries to AutoDocEntries.g
Moved Create doc entries to new file.
Moved main doc file to new file.
Modified CreateAutoDoc for AutoDoc itself.
Enabled AutoDoc for AutoDoc.
Added new documentation files.

I left the GAPDoc code in the files nevertheless,
because it might be useful if someone looks in the
sourcecode. They will not be maintained any more from now.
Merge branch 'create-page' of github.com:fingolfin/AutoDoc

* 'create-page' of github.com:fingolfin/AutoDoc:
  misc: reduce some minor code duplication
  Create{Title,Main}Page: Some TODO comments
  CreateTitlePage: also show maintainers
  CreateTitlePage: Introduce OutWithTag() to improve output
  Improve Create{Main,Title}Page
  Shorten AUTODOC_XML_HEADER, remove trailing whitespace in it