Generate Markdown docs for cosign

[imjasonh]

Summary

Now that #706 is closed (👏 @n3wscott), we can use cobra's built-in doc generation to make Markdown docs available for cosign.

Example: https://github.com/imjasonh/cosign/blob/gendoc/doc/cosign.md

This makes it easier to reference CLI surfaces in docs and in conversation.

This also adds a CI check to ensure new surface changes are accompanied by doc regeneration.

Open Questions:

• Do we also want to generate docs for sget, which also lives in this repo? This might change where we want generated docs to live.

Ticket Link

n/a

Release Note

cosign has generated Markdown docs available in the doc/ directory.

[n3wscott]

That is really awesome!!!