Add document for plugin install feature

[VinozzZ]

We need to have a doc page that explains how to configure plugins through agent config. It should also points out where are the default values are set in both the controller code and the installer bundle

[carolynvs]

We would like a dedicated page for configuring the porter agent, like a administrative task page. If we are inspired a concept page that explains what the porter agent is too.

[carolynvs]

We need to document how "latest" and not specifying a version impacts how long a porter agent volume is cached and how that can cause problems.

See #134 which would address the problem. For now we at least need to document it.