Change all docs filenames to .adoc

Change all docs filenames to .adoc

This commit renames all AsciiDoc files to have the ".adoc" extension.
Also, this commit updates all "include::" references in the root.adoc
file.

RATIONALE: Some editing tools, including GitHub, will auto-generate an
HTML preview for AsciiDoc files if they have the filename extension
".adoc" or ".asciidoc". The community agreed to this change in 2015 (see
http://markmail.org/thread/z2s7xnxavpjzirwx).

NOTE: The docs build script will need to change the reference from
"root.txt" to "root.adoc".

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