docs(*): update CONTRIBUTING.md and create DEVELOPERS.md

CONTRIBUTING.md
- focus on basic info about issues and pull requests for new contributors
- move development heavy info to DEVELOPERS.md + add links
- remove outdated info

DEVELOPERS.md
- contains info about project setup, coding rules, and commit message guidelines from CONTRIBUTING.md
- add and update info about writing docs from Wiki
- add info about development setup from docs contribute.md
- add info about running tests on Saucelabs / Browserstack

Closes #7303
Closes #9444
Closes #16297