Prepare for splitting up Getting Started docs
Split Getting Started docs into separate sections

The "Getting Started" docs became overly long and unwieldy over time.
This splits it into separate sections, including:

* Quickstart
* Installation
* Writing content
* Publish your site
If PATH is undefined, Pelican uses PWD as content
Merge pull request #1345 from justinmayer/docs34

Split "Getting Started" docs into separate sections