Switch makedoc.g to use AutoDoc

[fingolfin]

Use this to avoid duplicating the package version and release date in
doc/guava.xml, removing one potential source for mistakes when doing
releases.

The main advantage is that it removes the hard coded path to a GAP
executable, and also works correctly with ReleaseTools.

[fingolfin]

@osj1961 it would be great if you could either

1. merge the three open PRs for guava and make a new release of it (a lot of changes have accumulated since the last release), or
2. give me permission to merge them and make that release (it'll take me about 5 minutes of work overall to do that, so no problem for me at all)

[osj1961]

Max, I've merged all three. If you really don't mind doing the release, I'd be grateful. -Joe

[fingolfin]

Done.