Move CI that builds the demos for demos.haxeflixel.com to this repo

[ninjamuffin99]

Currently the CI that actually builds the demos for the site is at https://github.com/HaxeFlixel/demos.haxeflixel.com

which is a repo dedicated to just posting the demos for use with the haxeflixel site. I think we can instead move that stuff to this repo, and get the demos.haxeflixel.com github pages setup on this repo.

[Geokureli]

I think i keep forgetting to build demos because it's a separate repo, so yeah this would be simpler