Refactor testing docs

[Ramiro Morales]

Currently we have only one document that all overview, in-depth and reference topics that IMHO is too bloated.

I've refactored it a bit so reading of the main document can be performed in a more natural way without so many distractions.

Changes include:

• Move introductory material to a simple document (index.txt)
• Create a advanced document and move there description of
  • RequestFactory
  • API for custom test runners and integration with other testing frameworks/coverage.py.
  • Advanced multi-db testing topics.
• Move the doctests documentation to its own document.
• Add links to the new testing tutorial and among the documnts
• In the main document: Reorder the TestCase classes sections following their OOP hierarchy. Make titles nesting less deep.

PR is at: ​https://github.com/ramiro/django/compare/testing-docs-rf