Integrate and extend documentation for "documentation" and "xdoc"
Transition away from arithmetic-2.

 - Switched all books that were still using arithmetic-2 to instead
   use arithmetic-3, except for:

     projects/concurrent-programs/bakery/stutter2.lisp

   because some proof broke when I tried it and I didn't try to
   debug it.  (It might be easy, or not.)

 - Stop building arithmetic-2 as part of the "basic" target since
   it isn't so important.

Just getting rid of arithmetic-2 altogether seems like a better plan
than to try to resolve its name clashes with, e.g., arithmetic-3.
switch last book away from arithmetic-2
Fix acl2(r) incompatibility after arithmetic-2 redux
Fix acl2(non-h) incompatibility with vl server
Adding a reference to centaur/fty/basetypes from deffixtypes topic.

Indeed, it's kind of lame that this reference will need to be maintained, but I think the benefit outweighs the risk.
Merge remote-tracking branch 'upstream/master'