Rewrite after design discussion

* Run `update-toc`
* Update kep.yaml
* Several items fixed
* Update toc
* Rewrite Summary and Motivation sections
  - Start on Proposal section as well
* Revise Proposal section
* Update sequence diagrams
* Update risks and mitigations
* Add changes to TokenRequest API
* Update authz checks section, and all prior sections
* First pass of Design Details section
* Start updating test plan
* Pass through PRR questionnaire and alternatives
* Remove TODOs and add more specific links
* Update troubleshooting
* Low-hanging fruit
* KAS dedicated service account name: open question
* Proofreading pass
* Changes after review
* Remove inaccuracy

Signed-off-by: Peter Engelbert <pmengelbert@gmail.com>