#2549 - Split 'generating a new extension' into its own guide (from the Getting Started guide)

Issue 2549:  Split the "Getting Started" guide into 2 guides by pulling section 6 "Generating a new extension..." out into its own guide.

(I haven't done a pull request on github before.  I didn't squash my commits; I wasn't sure if you'd handle that or if I should.  Just let me know if I need to correct something or do something differently! )