Significantly restructure and embellish the review process documentation.

- Generalized some of the early wording to not just talk about the language and standard library
- Moved the list of covered evolution areas from the Scope section to a later and more comprehensive Evolution Areas section
- Unified the discussion about what is and is not covered by the evolution process into the early "scope" section
- Changed some of the wording around when bug fixes are effectively design changes
- Turned the Community Structure section into an Evolution Workgroups section:
  - Moved earlier in document.
  - Removed list of specific workgroups (now covered comprehensively by Evolution Areas).
  - Added brief discussion of Core Team delegation.
  - Added text giving workgroups authority for deciding how the process applies in their area.
  - Added text describing how cross-area proposals should be managed.
- Added an Evolution review section:
  - Added a quick summary of the review process (proposal development + open review).
  - Permit summary judgments, with caution.
- Greatly expanded Participation section:
  - Added extensive guidance for participating in review.
  - Moved "How to Propose a Change" section into this section. Extensively reworded guidelines.
  - Moved discussion of focus areas to a separate section near end of document.
- Added Evolution Areas section:
  - Describes three existing evolution areas, their workgroups, and the details of the evolution process used in each area
  - Fixed the description of package manager proposals, which the current document still assigns to the PSG instead of the ESG.
- New Focus Areas section:
  - Removed from proposal guidance
  - Weakened wording to reflect actual practice
- Changes to final sections of interest primarily to workgroups:
  - Moved the proposal status flowchart into this section
  - Added a section about generic proposal document structure: required fields, etc.

Author: rjmccall