Docs

[tmcgilchrist]

A couple of minor fixes for documentation:

• https://github.com/mirage/prometheus/pull/48/commits/9e8a4047ad72e9edcfd520532996f492ff8f3a20 makes the prometheus links actual HTML links and adds missing module docs.
• https://github.com/mirage/prometheus/pull/48/commits/133ed73c56337ff6304073a164e92d109bf66c22 adds a small example of how to use cmdliner and prometheus_unix from ocaml-ci

[tmcgilchrist]

@talex5 I agree it does not work as is, I will make it into an MDX checked version and resubmit a different PR.

[tmcgilchrist]

https://github.com/mirage/prometheus/pull/50 just the docs fixes.