Review and restructure appsody documentation

[neeraj-laad]

Appsody documentation has grown since the first release, but some of the docs are very large and difficult to follow, while others are too trivial and miss advanced topics/sections.

It would be great to review the overall structure and come up with something that lets us put simple flow across to new users (like application developers) but also let advanced users (like stack providers or repository maintainers) get more detailed information of how they can utilize other appsody features to suit their requirements.

[SueChaplain]

@helenmasters - shall we create some issues to break down the actual doc updates, starting with the install section? Be easier to size the effort and work out how we can get it done as quickly as possible.