Add component example/starter code

[kbx81]

Update and incorporate into developer docs (backlog/#2) examples from https://github.com/thegroove/esphome-custom-component-examples

• Create repo (done: https://github.com/esphome/starter-components)
• Update example code to build & run on the current ESPHome release
• Expand the examples to include some actual I/O - Add some example I/O starter-components#4
• Review & polish example code
• Add into developer docs - Add link to starter components repo to home page developers.esphome.io#41