feature: Documentation command

[xNok]

I found myself wanting to generate reference documentation for my policies, and I figured it would be nice if this was a built-in feature of conftest.

While the way of structuring the documentation can be opinionated, I think using the FlatAnnotationsRefSet is a safe bet, as OPA's core does all the heavy lifting of organizing annotations and rules.

My initial attempt was to combine FlatAnnotationsRefSet and Go-template and offer the least opinionated approach to documentation format that can be easily overridden. However, I had some trouble properly formatting titles (H1, H2, H3, etc...) and had to introduce some custom logic to handle it. (As a side note this could potentially be handles by adding template function libraries such as sprig)