Add github action to update command reference pages

[popey]

The pages in content/docs/reference/commands/ show the command line help for various commands, including Syft and Grype:

We should automate the creation of these, so they're up to date, and easily maintained. These are simply reference documents that allow potential new users to see the output of our commands without having to install them.

There's also the benefit of having these up to date, for search engine and generative engine optimization.