Articles/issue 122 fluent configuration

related to #122.

**this is a very first draft.**
I've found a lot of documents regarding the configuration API all unrelated and not linked to each other.

* `fluent-config-api-v3-v4-intro` aims to be the hat on top of all the others.
* `fluent-config-api-v3-v4` is the entry point that details all the available API and links to all the detailed articles.

in the latter below the line at the end there are a couple of things I do not know how to handle.

Call to action for @dannycohen @SimonCropp @indualagarsamy and @johnsimons: comments and review :-)
Should it be Func<string> or Func<String>?
First overload is using `string`.