Hydrogen-ui storefront API schema and types

[frehner]

Closes #1025 (and checks off an item on my list) and adds very basic intro docs on how to implement these features.

Relies on #1115 to be merged first, since it builds on that PR. (Opened this PR against that PR so that this one doesn't look as big, I'll change the target after 1115 is merged)

One thing I'm still thinking about is the best way to make the storefront TS types accessible to hydrogen; currently, I think our choices are

1. Duplicate the TS types and graphql generation code (not a fan)
2. Force hydrogen to depend on hydrogen-ui and use the types just like any other library/store

Other ideas?

[github-actions]

We detected some changes in packages/*/package.json or packages/*/src, and there are no updates in the .changeset.
If the changes are user-facing and should cause a version bump, run "yarn changeset add" to track your changes and include them in the next release CHANGELOG.
If you are making simple updates to examples or documentation, you do not need to add a changeset.

[frehner]

@cartogram What we talked about this morning

[blittle]

LGTM, I'd like to ship an unstable version of this after merge and test it out.

[frehner]

LGTM, I'd like to ship an unstable version of this after merge and test it out.

Hydrogen-ui isn't publishing yet, but that's on my todo list this week.