Fixes for auto-generation of man pages

stg.txt is the only man page not generated by stg-build --commands.
Provide this page explicitly instead of including all other
commands twice.

stg-cmd-list.txt is only a template for inclusion in other man
pages, so rename to stop it being built along with other stg-*
files.

Signed-off-by: Daniel White <daniel@whitehouse.id.au>
Signed-off-by: Karl Hasselström <kha@treskal.com>