Set up a 'building' docs folder

[cogat]

For containing information about building a GLAMkit site. It should contain information about creating and deploying a GLAMkit site. Specifically:

• The current overview/first-site.rst
• The doc in Document updating a local copy #277 and Document how to use S3 media locally #280.
• How to create a new Rich Content model.
• How to create a new Page Type
• How to create a new Content Plugin
• The contents of deploying/index.rst.