magic: refactor a bit, add new func AUTODOC_FindMatchingFiles
magic: allow passing optional args to AutoDoc() via options
magic: rename autodoc.files_to_scan to autodoc.files

This matches gapdoc.files more closely.
magic: derive bookname from package_info.PackageDoc.BookName

If this is NOT present, then warn the user about this and suggest
how to correct this.

Note that it is not anymore possible to specify the bookname via a
parameter to AutoDoc(). Indeed, if PackageDoc.BookName is missing,
then GAP will not see the manual anyway, rendering the bookname
irrelevant in this case.
cleanup
doc: complete (mostly) the AutoDoc() documentation

I cheated a bit and commented out a few thing like scaffold.entities,
see also the corresponding TODO comments.
magic: gapdoc.main may already have ".xml" suffix, deal with that
doc: expand tutorial a bit
Removed Labels, renamed FunctionLabels to labels.
makedoc.g: change AutoDoc() call to use options
doc: add chapter on doc comments, based on Sebastian's notes
doc: tweak tutorial a bit, link to comments chapter
Merge branch 'devel' of https://github.com/fingolfin/AutoDoc

* 'devel' of https://github.com/fingolfin/AutoDoc:
  doc: tweak tutorial a bit, link to comments chapter
  doc: add chapter on doc comments, based on Sebastian's notes
  makedoc.g: change AutoDoc() call to use options
  doc: expand tutorial a bit
  magic: gapdoc.main may already have ".xml" suffix, deal with that
  doc: complete (mostly) the AutoDoc() documentation
  cleanup
  magic: derive bookname from package_info.PackageDoc.BookName
  magic: rename autodoc.files_to_scan to autodoc.files
  magic: allow passing optional args to AutoDoc() via options
  magic: refactor a bit, add new func AUTODOC_FindMatchingFiles