Give the guide some content

[fitzgen]

The guide is pretty much empty. It should have something in it.

---

Straw proposal:

• Make every gloo crate use cargo readme to generate its gloo/crates/whatever/README.md from its module documentation

• Use mdbook's file include feature to include these as a section in the guide

This would get us a good amount of documentation in all the places users expect it to be, without multiplying the maintenance effort.

Thoughts?

---

+cc @David-OConnor @Pauan @yoshuawuyts @rylev

[Pauan]

That sounds fine to me. I agree that the guide's contents should probably match with the crate's docs.

[David-OConnor]

The current crates are fairly straightforward, and are easy to understand from the comments in code and examples. The guide will be more relevant once complex, interacting crates are included.