Switch from JSON to YML for data files.

Switches data files from JSON to YML format. YML is easier for non-developers to use.

Documentation related to this change:

1. [Building for non-developers](https://github.com/optimizely/marketing-website/wiki/Building-for-non-developers)
2. Added link to above wiki page on [Assemble pages](https://github.com/optimizely/marketing-website/wiki/Assemble-pages).