Move introduction to about

mkdocs, aws and net-ness all want an /index.html.

So I moved the intro-intro section document to 'Home', and renamed the 'Introduction' section heading to 'About' - else we have lots of different items called 'introduction'

Imo, these 4 docs are much more useful than 'introduction' suggests - especially given that we have introduction headings in many of the docs, and that it can be mis-read as introducing the documentation, when its really giving a grounding basis information for new users.

try it out at: http://mkdocs-about.s3-website-us-east-1.amazonaws.com/