MFC r325834,r325997,326502: Move sys/boot to stand/

This is effectively a direct commit to stable/11, due to differences between
stable/11 and head. Changes to DTS in sys/boot/fdt/dts were often
accompanied by kernel changes. Many of these were also risc-v updates that
likely had many more dependencies to MFC.

Because of this, sys/boot/fdt/dts remains as-is while everything else in
sys/boot relocates to stand/.

r325834: Move sys/boot to stand. Fix all references to new location

r325997: Remove empty directories.

r326502: Document the sys/boot -> stand move in hier.7 and the top-level README.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

mdoc: improvements to SEE ALSO.

Fix domain.

Approved by: emaste

Fixes to man8 groff mandoc style, usage mistakes, or typos.

PR: 168016
Submitted by: Nobuyuki Koganemaru
Approved by: gjb
MFC after: 3 days

Update contact info.

PR: N/A (mentor approved)
Submitted by: dteske
Reviewed by: dteske (mentor approved)
Approved by: emaste (mentor)
MFC after: 1 week

New boot loader menus from Devin Teske.
Discussed on hackers and recommended for inclusion into 9.0 at the devsummit.
All support email to devin dteske at vicor dot ignoreme dot com .

Submitted by: dteske at vicor dot ignoreme dot com
Reviewed by: me and many others