Put "ditamap" in italics and in lowercase

- put "ditamap" in italics, except when belonging to sentence in italics
- put "ditamap" in lowercase
- remove :file: directive for ditamap file type
- remove period before "ditamap" when not used as file extension

Signed-off-by: Olivier Carrère <carrereo@gmail.com>
Acked-by: Angela Cristina <acristina@redaction-technique.org>