New search docs

Primary goal of this PR is to clarify the re-indexing documentation for 5.0.3. A good deal of work went in to making the process faster and more surefire. Will a user who's been warned of the need to re-index (via troubleshooting and the release announcement) be able to follow the outlined steps from top to bottom and have a clean&fresh search index? Does any of the terminology need to be further explained? Etc.

Remaining tasks:
 * [ ] Introduce a link to the upcoming metrics docs
 * [ ] Update the nightshade statistics (@kennethgillen)
Problem here in Sphinx syntax: the ``term`` is not recognised as term in html
![screen shot 2014-07-16 at 11 11 30](https://cloud.githubusercontent.com/assets/2478303/3597430/a7729212-0cd1-11e4-8f1a-0399501e252e.png)