[docs] Refactor "Developing a Runtime" docs and add `startDevServer()` (#4675)
[docs] Refactor "Developing a Runtime" docs and add `startDevServer()` (#4675)

Co-authored-by: Steven <4068f0880b399410602d694b3cc711c8a8f4727e@ceriously.com>
zeit/now-cli,zeit/now-cli,zeit/now-cli,zeit/now-cli,zeit/now-cli,zeit/now-cli,zeit/now-cli