Docs: Apache config includes examples for "no image"

Docs: Apache config includes examples for "no image"

This commit adds a note about the new commented config blocks in the
example Apache vhost file for providing a custom image for items with no
image. This commit also moves a related section into the "External
Content" section, so all external content info is together.

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