New search docs (rebased onto develop)


This is the same as gh-870 but rebased onto develop.

----

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 (for another PR):
 * [ ] Introduce a link to the upcoming metrics docs
 * [ ] Update the nightshade statistics (@kennethgillen)
 * [ ] clarify the online/offline difference (@sbesson)