Docs: Fix leveloffset bug, raise REL NOTES level

We should restrict our use of the asciidoc leveloffset command to the
root.txt file, since it is a global setting and cannot be set relative
to an existing offset.

This commit also raises the Release Notes section to the highest level
in order to prevent the lowest level subsections within it from being
lost. This is in response to asciidoc errors:

    WARNING: RELEASE_NOTES_2_5.txt: line 950: missing section: [sect5]

Signed-off-by: Remington Steed <rjs7@calvin.edu>
Docs: Remove ref to missing included file

This file was moved into the 2.5 release notes. If others believe it
belongs in the main documentation, it can easily be added in again. For
now, we will remove the reference because it only generates an asciidoc
warning.

Signed-off-by: Remington Steed <rjs7@calvin.edu>