Keystone: Add docs for `create-keystone-app` CLI args

[jesstelford]

New CLI args were added in #2273 - we need to document them.

[gabipetrovay]

@jesstelford Let me give you a PR for it as well.

Do you know if changesets are also required for documentation updates? I see that there would be at least 2 packages changed with this update: the create app (for all arguments) and some guides updates with code samples. Does it sound right?

[jesstelford]

If it's purely docs, no changeset is required (as we don't "release" the docs), but if it includes changes to a package's README, etc, then we usually just add a patch bump to ensure the docs make their way to npm.

[jesstelford]

Actually, I take that back; pure-docs changes should do a patch bump for the @keystonejs/website package. I don't think it requires it, but good to be consistent.

[gabipetrovay]

And another question on my previous changeset.

When I created it, I bumped the minor version. But I still see in the repo that this package still has the old version (1.4.1) instead of a bumped (1.5.x) what I was expecting. Do you know if this package.json will be updated by some other tools (upon release for example?)