Update Package Development documentation

I was feeling a little frustrated and embarrassed at the state of our package development docs.  On the mailing list today, a new-ish developer was directed to the docs to figure out how to tag and publish a package update.  While it's possible he could have done it using `git`, there is actually very little guidance in the docs on how to update it at all.

This PR cleans up the Package Development section, promoting it up one level, dividing it up into subsections, fixing up and filling out the examples, and reorganizing it slightly.

I'm hoping this PR either makes Stefan's life easier for the promised update to the `Pkg` docs, or goads him into doing it. :-P

Cc: @StefanKarpinski