LP#1848524: Docs: Delete unused assets

LP#1848524: Docs: Delete unused assets

This commit attempts to delete all images from the media/ directory that
aren't used within an AsciiDoc file (via "image:" or "image::"). This
commit also doesn't touch images that are known to be used in the HTML
template (such as the Evergreen logo, PDF/ePUB icons, NOTE icons, and
callout number icons).

This list of images was generated via multiple bash commands, grepping
and diffing. It would be great for us to have a standard command for
finding these periodically.

This commit also removes all AsciiDoc files that we don't currently include
in another AsciiDoc file (via "include::"), because that means they
never appear in the generated docs (HTML, PDF, or ePUB). Some files have
been left untouched, such as everything in the TechRef/ directory. It
also further removes any images referenced by these files.

Signed-off-by: Remington Steed <rjs7@calvin.edu>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>