Create an overview section in doc

[elsony]

On the front page of the web page, we should have an overview session to describe the basics of devfile, e.g. what a devfile is. This will be the first piece of information that the user can read when they are interested to find out what a devfile is. We can move some of the background information from the stack author guide (https://devfile.github.io/devfile/assembly_making-a-workspace-portable-using-a-devfile.html) to that overview section.

[jc-berger]

Closing as the PR addressing these concerns have been merged.