Improve Runway customization doc
What
This MR documents how Introduce service-specific runway.yml (runwayctl!81 - closed) and Allow Service project to customize CD workflow (ci-tasks!49 - closed) work.
Why
Currently, the documentation lacks the customization sections. Service developers can't understand what options are available.
Related to Allow Service project to customize CD workflow (team#36 - closed)
Edited by Shinya Maeda