Automate CLI Usage Docs

We should probably automate the `thv` usage docs like [`cosign` does](https://github.com/sigstore/cosign/blob/main/cmd/help/main.go) [here](https://github.com/sigstore/cosign/commit/371845b023faca47f609b9547ba4d02d8db9673a) . An example of what it generates here https://github.com/sigstore/cosign/blob/main/doc/cosign.md.

This alleviates content from the README.md as we can just point to it.