Change Makefile default to "basic", documentation updates

Documentation updates

  - Integrated many changes from centaur/doc.lisp into
    system/doc/acl2-doc.lisp, reducing much of xdoc::fix-the-hierarchy

  - Add mention of note-6-4-books to the start of note-6-4

  - Slightly reworked :doc BOOKS

  - Change various links to avoid using URLs as the link text.  (As a general
    principle, I think we should generally try to avoid "Click Here", URLs, or
    other non-descriptive link text.)

  - Other minor/trivial edits.

Update issue 110

I reworked the documentation at the start of books/Makefile and made
"basic" the default.  I expect there to be some fallout from this.  I
think we minimally need to:

  - Decide how we want to document the acl2 books build procedure and options.
    Currently we have instructions in at least the following places:

      1. The ACL2 web site installation guide
          - under "Easy Install for Unix-Like Systems"
      2. The "Using ACL2" web page about "Certifying ACL2 Books"
      3. The BOOKS-CERTIFICATION topic
      4. The CertifyingTheBooks page on the Wiki

    I think we should decide on a single place for these instructions to be,
    then update all of the above accordingly.  (I'd propose putting
    instructions into ordinary xdoc topics like BOOKS-CERTIFICATION; if
    there's no objection I'm happy to write a first cut at this.)

git-svn-id: http://acl2-books.googlecode.com/svn/trunk@2271 303c5b2c-9740-0410-b014-fd8f9db7be32