Make readmes, configs, templates, etc more DRY

[iandunn]

WordPress/wporg-news-2021#10 (comment)

@iandunn: Adding [cli script] to repo-tools would be nice too. That goes for some of the other suggestions too. I wonder if there's a more DRY & automation-friendly way than forking repo-tools/templates/*/ 🤔

Can scripts extend the scripts from repo-tools, or something like that?

Maybe /README.md can just link to /vendor/wporg/wporg-repo-tools/readme.md for the common stuff? Or probably the github.com version, so they can see it before installing. /env/README.md could do something similar.

@coreymckrill: It also might make sense to wait until we have multiple project repos depending on it before we add a bunch more stuff to it, so we have a better sense of what's needed.

I think that makes sense for the stuff that's unclear, but isn't it safe to assume all child themes will have a README with setup instructions, a header template, phpcs config, etc?

Related: #2

I put this in the 1st repo milestone, but we can move it to 2nd repo if we decide to do it then.