rename CACHE_DIR -> CACHE_PATH to unify with rest of Pelican

CACHE_PATH can now be relative to settings file like OUTPUT_PATH.
Also add --cache-path commandline option.
Change cache loading warning to a less scary and more helpful message.
Fix #1335 remove old and unmaintained French docs

Unfortunately nobody keeps them up to date, they would just spread
confusion.
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
Fix RstReader authors metadata processing

The reader would return a list of authors already, but
METADATA_PROCESSORS['authors'] expects a string.
Added a test case for this (only the HTMLReader had it).