build: generate man page list from source files, not conf.py

Use $(wildcard ...) to generate the list of man pages based on the rst
source files present in the man page directories, instead of reading
conf.py. This has three main benefits:

1) This makes the man page build slightly less complicated and easier
   to understand. At least there are fewer moving parts.

2) This makes the build fail if we add a man page rst file, but fail
   to add it to conf.py.

3) We can use Sphinx constructs in conf.py that are not available when
   importing the file into a normal python program such as
   mkdocdeps.py.