Write an introduction for the core howto docs

[exarkun]

The core docs start off with three "introductory" documents. The vision statement, the "overview", and the "internet overview". These are all rather out of date and are pretty short on the sort of material which usually appears in the introduction to an unusual, expansive system such as Twisted. Specifically, the introduction should address:

• What Twisted is good for
• A high-level overview of how Twisted works
• How the documentation is laid out (inasmuch as it follows any sensible layout, which unfortunately isn't very much), including the split between api docs, howto docs, and examples

And (so that I can resolve #4136) there should also be a mention of where the latest version of the documentation can be found.

[khorn]

This  blog series has some great information that could possibly be adapted to this purpose.