documentation: Updates to support developing 2.3 YP docs

documentation: Updates to support developing 2.3 YP docs

Changes to the following:

 * All manual revision tables to indicate a 2.3 release proposed
   for April of 2017.

 * Updated the poky.ent file so that all variables needed reflect
   2.3 values.

 * Updated the mega-manual.sed file so that processing of links in
   the mega-manual work for a 2.3 release.

(From yocto-docs rev: 926e282c17b67c79b94a0b62a0b18bfbe0b26c40)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>