Port various documentation topics to xdoc.
start of tour stuff and doc updates
second recovered patch from old git
fix autohide bug
port documentation for easy-simplify to xdoc
port untranslate-patterns documentation to xdoc
Merge remote-tracking branch 'remotes/origin/master' into doc
Changing NONSTD system build target to ACL2_REAL.
Merge pull request #257 from harshrc/master

ACL2 Sedan changes (Cgen and Defdata)
Merge branch 'master' of https://github.com/ragerdl/acl2
Changed NONSTD to ACL2_REAL in GNUmakefile, anticipating changes about to be made by David Rager to the books.
Merge pull request #259 from harshrc/master

sig is now just defdata::sig
Merge remote-tracking branch 'upstream/master'
Fixing a fix of the hierarchy
Updating some data-definition references.
Defdata is its replacement
Removing change-parents calls for some cgen
topics.  We might need to reinstate some of them (perhaps under the
topic cgen if it exists, but I'm just trying to get the build working
for now.
Merge pull request #260 from ragerdl/master

6 CLs
Fixes to without-interrupts (GCL only), :doc sys-call, two loop index types in memoize-raw.lisp.  Renamed defdata-regression.lisp to .lsp.

Fixed gcl-only bugs in without-interrupts.

Updated :doc sys-call to clarify (as per IRC channel discussion) that
one cannot expect shell expansion of arguments.

Improved types from integer to mfixnum on a couple of loop indices in
memoize-raw.lisp.

Renamed a books/defdata/defdata-regression.lisp to a .lsp suffix, as
suggested by Harsh (since it's not a book).

Updated doc.lisp to match books/system/doc/acl2-doc.lisp.

Changed "SINCE" date stored in bin/purity.sh to avoid bogus error (my
changes to GNUmakefile had been the same as David Rager's).
Improved an error message for certify-book (thanks to feedback from David Rager).  Removed obsolete file p.lisp.
Improved some comments and tweaked organization in memoize-raw.lisp, through chat with J.  Defv is now defvar in non-CCL lisps (instead of defparameter).