Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC r250746 and r250805:

- Update the release notes to DocBook 4.5 XML to build with the
updated documentation tree
- Fix build after doc/projects/xml-tools integration.

Add a document for brief installation instructions.

Submitted by: kensmith

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Disconnect installation notes from the release documentation build.

Approved by: re (blanket for installation notes removal)

Retire the Early Adopter's Guide in HEAD.

Disconnect early-adopter/ from the build.

Hook up the Hardware Notes to the build again, now that the doc build
script has been updated.

Temporarily unhook the English Hardware Notes from the build to unbreak
the web build.

Pointy hat to: simon

Hook up early-adopter article to RELNOTESng build.

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