Publish demo from /docs instead of gh-pages branch

[stefcameron]

This would simplify the publishing process, and make it possible to update the demos in PRs.

• Rename demos dir to docs
• Update GitHub settings to publish from master branch on /docs dir
• Update demo publishing instructions to be that a local build should be run so that it updates /docs/bundle.js as part of a PR
• Change build so that bundle.js is generated into/docs/bundle.js
• Remove bundle.js from .gitignore
• Add "Run build to update bundle.js if a demo has been altered" to PR Checklist