Host docs for PRs

[Alizter]

I think it would be beneficial for the CI to deploy docs from PRs so that PR authors can easily inspect the result without having to build the docs locally, which can require configuring CSS, or having to wait for the PR to be merged into master.

[jdchristensen]

It seems to work fine for me to build the docs locally and just view them in a browser, with no server. Maybe we just need to document this in STYLE.md?

[Alizter]

I think maybe alectryon had some weird CSS issues then I don't recall. The point is we ought to be able to see it all from the CI.

[Alizter]

I think documenting it in STYLE.md would be a good idea. I'll do that.