Docs: integrate holds docs from EG 2.1

Docs: integrate holds docs from EG 2.1

The small amount of Holds docs present in master seemed to fit well at
the end of the older Holds documentation, so this commit appends them
to the new holds.txt file, deletes the previous holds_management.txt file,
and makes the appropriate change to root.txt.

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