Implements `make docs` command to create rtd-themed docs using Sphinx
from the existing Markdown documents in the docs directory.
Changes the `get-modules` command to `depend` to more closely match
OpenStack guidelines in [0].
Adds `fmt` command as specified in [0], currently referencing the `lint`
command.
Adds `godoc` command that will host a local godoc server instance with a
link directly to airshipctl technical documentation.
Adds `releasenotes` commands specified in [0], currently
noted as "TODO".
Adds `images` command as found in other Airship projects, referencing
`docker-image`.
[0] https://governance.openstack.org/tc/reference/pti/golang.html
Change-Id: I134fc2c2714bce6d23dc9cf8b86c3016d68def7e
b8a2bb3b27