Issue2549 split extension out of getting started guide

/doc/guides/1 - Getting Started files changed: pulled the section on extensions out of the file "2 - Getting Started" into its own guide ("3 - Generate an Extension to Use Your MVCs").  Incremented the number in the file name of the other files in that directory.  Clarified "extension" and "engine" with some additional sentences. Fixed a minor spelling error.

I'm not sure if the URIs I put in to other guides are correct.  I looked at the other .textfiles to see how they did it.  Be sure to let me know if they're not OK.

(And thanks for your patience.)