Reorganize and expand quickstart big picture docs on environments

* Add Environments section under "Understanding the Fundamentals" to introduce environments. This uses the glossary definition as a starting point and expands on it a little.
* Move the Web Profiler discussion out of Working with Environments into a separate section, and add a paragraph briefly summarizing what the profiler can do.
* Improve the Working with Environments section.
* Original changes from PR #2683 rebased on 2.2
Big Picture Environments cleanups

* Move the environments and front controller discussions to book/page_creation
* This allows for the debug toolbar discussion to be more prominent
Changes for correctness
Merge pull request #2823 from jbafford/big-picture-environments-22

Improve Quick Tour "Big Picture" section and expand Environments docs