docs(CONTRIBUTING): update and improve

- remove outdated info
- add info about writing docs from Wiki
- add info about development setup from docs contribute.md
- add info about running tests on Saucelabs / Browserstack

TODO:
- add link to contributing.md to Contribung page
- add link to changelog.md to dropdown in docs
-- link to current version changelog entry?

Closes angular#7303
Closes angular#9444

Author: Narretz