dillon1000/react

Add 'Test Utils' docs back to main navigation (#9676)

* 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* Add back the shallow renderer docs and remove outdated docs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- Remove outdated 'shallow renderer' docs on 'test utils' page, and point to the updated 'shallow renderer' docs.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