Consolidate reference documentation

[ealmloff]

The documentation under the reference heading is generally more out of date and less detailed than what we have in the inline docs on docs.rs. For the documentation that covers a specific higher level concept, like the rules of hooks, we should move it into the essentials or guides section. For documentation that covers a specific hook like use_coroutine we should remove the page and point people to the docs.rs reference instead

We could also embed the docs.rs documentation into a subdomain on our docsite like web-sys does. Our search indexing should be compatible with any html file so we could make the docs.rs reference pages show up in the guide search results as needed