Fix intrasite links substitions in content

The Content.__eq__ method would indirectly call _update_content too
soon, resulting in failed intrasite links substitution
This effectively reverts fd779267000ac539ee0a9ba5856d103fbbc7cd7c
for pelican/contents.py, it was unnecessary anyways.
Thanks to Strom for finding this.
Fix typo in command-line option description.
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
move {ARTICLE,PAGE}_DIR -> {ARTICLE,PAGE}_PATHS

Instead of one path a list can be given. This is due to popular request.
Should help people not wanting to use Pelican for blogging.
Maintain backward compatibility though.
Thanks to @ingwinlu for pointing out the change in StaticGenerator.