Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

- Rename .sgml files to .xml to track the recent doc changes

MFC after: 1 day

Fix DESTDIR.

Spotted by: bmah

Fix a build breakage due to undefined DESTDIR.

Reorganize the release notes to better support multiple architectures.
Rather than rendering a different version of the release notes for
every architecture, we now produce a single release notes document
that covers all architectures.

This change makes document maintenence easier (because there is no
longer a need to support each architecture specifically in the
src/release/doc/ and www/ build infrastructures). It's also
easier to read because there is now exactly one release notes document,
and readers can see changes that affected various subsets of
architectures.

Discussed on: -doc@

Add the bones for PowerPC release documentation. This has been
derived from ia64.

Remove the rest of the alpha-specific MD release documentation
(release notes and installation guide).

Commit amd64 MD release documentation stubs and hook up to the build.
Content will be filled in later as needed, but this should unbreak
the doc part of an amd64 release build.

Requested by: peter

Connect the ia64 versions of the release documentation to the RELNOTESng
build.

This will give ia64 folks the chance to help flesh these out, and
also ensure that (when the time comes) ia64 releases with documentation
won't fall over due to missing RELNOTESng files.

Add/Update hardware, installation and relnotes documentations for pc98.

Submitted by: NAKAJI Hiroyuki <nakaji@tutrp.tut.ac.jp> and nyan
Reviewed by: bmah

Begin building sparc64 release notes for -CURRENT.

Build system:
o Define a RELN_ROOT variable which points to the root of the relnotes
tree (i.e., src/release/doc).
o By default, define DOC_PREFIX in terms of RELN_ROOT; this gives a
bigger chance of finding the doc/ tree without help in the form of
setting DOC_PREFIX on the command line.
o Respect DOCDIR; `make install` works now.

Approved by: bmah

First commit of RELNOTESng, the rewrite of the *.TXT documentation
files.

src/release/doc/README has additional information.

Reviewed by: -current, -doc