PEP 817: add The Design Space part of the PEP, improve other sections

The largest addition is a side-by-side comparison of five design options, which
are the ones we've considered and those that have been brought up in the
various discussions on DPO. It's deliberately side-by-side, so reviewers can
understand how we arrived at the current design.

Note that in a call a while back, I showed a similar table to the one added
now, but with more decision criteria and explicit scoring (good, medium, bad
with emoji's); I decided against that because any scoring as well as the
choices of more detailed criteria can all be argued with, while the version in
this PR is as objective as possible.

Other sections that are updated:

- Abstract
- Overview of Standards Track PEPs
- Backwards Compatibility
- Security Implications
- Change History
- Adoption and Rollout (new)

Sections added or touched in the design overview:

- Depending on a variant-enabled package (new; this question has come up very
  regularly)
- Package ABI matching (needed a tweak)

The Motivation and Prior Art sections were not touched significantly.

Author: rgommers