Docs / Guides: Add a "how to publish to GHPages" guide

[davesmith00000]

Based on this commit:

https://github.com/davesmith00000/roguelike/commit/3683f4f46afabb2b1433947b221da734f258aa1e

And these instructions:

https://github.com/sbt/sbt-ghpages#initializing-the-gh-pages-branch