* Add 'Test Utils' docs back to main navigation
**why make this change?:**
We accidentally removed this - still supporting the use of Test Utilities, so we should have them in the docs.
**test plan:**
Manually tested the website - will insert a screenshot.
**issue:**
https://github.com/facebook/react/issues/9651
* Move test-utils docs to reference section
**what is the change?:**
Moved from 'advanced guides' to 'reference'
**why make this change?:**
It makes more sense as a reference
**test plan:**
Visual inspection (flarnie may add a screenshot)
**issue:**
* Add back the shallow renderer docs and remove outdated docs
**what is the change?:**
- Remove outdated 'shallow renderer' docs on 'test utils' page, and point to the updated 'shallow renderer' docs.
- Re-add a link to the updated 'shallow renderer' docs on the main navigation.
**why make this change?:**
This was already approved in https://github.com/facebook/react/pull/9331 which was then cherry-picked to https://github.com/facebook/react/pull/9359/commits and landed on master.
I'm not sure why some of these changes didn't persist. For now just adding back the changes we need.
**test plan:**
Manually inspected website - will insert screenshots.
**issue:**
* Further improvements to 'shallow rendering' and 'test utils' docs
Thanks @gaearon for the improvements!
**what is the change?:**
- Remove <hr/> from end of 'shallow rendering' docs
- 'documents' -> 'documentation'
- Move 'shallow rendering' redirection section to top of 'test utils' docs
- Add intro sentence about testing to 'shallow rendering' docs
**why make this change?:**
Documentation helps people learn.
**test plan:**
Visual inspection
Flarnie Marchan