Reorganize and expand quickstart big picture docs on environments

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.0+
| Fixed tickets | #2672

* 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.