Restructure the evolution process documentation

[rjmccall]

The focus of this change is on generally fixing and improving the documentation. For now, I am specifically trying to avoid resolving questions like whether SPM proposals should be part of the SE namespace and whether all evolution reviews should share a common category on the forums. The goal there is to document the current evolution process; if an evolution workgroup changes its practice in the future, editing this file will be the way it officially documents its new process.

An earlier version of this PR also laid out a lightweight evolution review process. I have removed this from the PR, not because I am uninterested in adding it, but because I want to land the general documentation improvements separately.

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 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.

[stmontgomery]

I think there's one small formatting issue outstanding (#2953 (comment)) but broadly speaking this is a great improvement to the process docs, thanks!