Generate command help from real output

[dnephin]

Right now docs/reference/docker-compose.md is a copy of docker-compose --help and all the docs/reference/*.md files that have the same issue.

We should be able to generate this output instead of copy-paste it into the docs. Trying to keep them sycned will result in the docs always being out of date (they are currently), and a lot of unnecessary work to attempt to keep them up-to-date.

I don't know much about https://gohugo.io/ but it's definitely something it should support. An example of this feature for sphinx can be found here: https://pythonhosted.org/sphinxcontrib-programoutput/

[albers]

Docker used to have a python tool docs-update.py for that purpose.
It was removed, though. Don't know the current solution.

See also moby/moby#9478.

[dnephin]

It looks like either gohugoio/hugo#796 or gohugoio/hugo#247 would make this possible.

Putting the content into an environment variable might also work until those are implemented.

I think another blocker is that the docs build only has access to the docs/ subdirectory, which means that it will need to change in some way.