doc: Merge substitutions into conf.py:rst_epilog

This file is only used there, it is short and keeping it away
introduces problems (warnings about .rst not being in any TOC entry,
:orphan: dangling if added, etc).

Change-Id: Iea33f22721856884aae9fc1dce46a2b90896286a
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>