Deploy mdbook in guide folder, preserve branch history

[flodolo]

This builds the book in a guide subfolder, and preserves the history of the gh-pages branch (in case of issues).

Fixes #366

[flodolo]

It's hard to tell if this is enough to fix the issue 🤔

On my fork I can see the pages on /guide, but is that enough?
https://flodolo.github.io/fluent/guide/

[eemeli]

This looks like it should fix the issue.

[flodolo]

Thanks, let's check in a couple of minutes 🤞🏼

I confess I'm quite confused on how projectfluent.org is set up.

[flodolo]

I can see the correct gitbook output in https://projectfluent.org/fluent/guide/ 🥳