Continuous Integration - Improve layout and add section for extension PRs
Created by: totten
Summary of changes:
- Emphasize use-cases first (e.g. "Proposal testing for core") -- and then drill-down to tools/specifics (e.g. Jenkins/phpunit).
- Re-style and copy-edit the section for core PRs. (ex: use
!!! tip
for distinct subtopics; present repo-test relations as a table). - Add a section for the latest work on testing extension PRs