Restructure the evolution process documentation

[rjmccall]

Summary of changes:

• 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 a General Principles 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.
• 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
• 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.