Refine CONTRIBUTING.md (#10324)
* Slightly refine CONTRIBUTING.md
* Write more about run_spec
* Wording
* Deeper tests is an advanced topic. Move to bottom
* Mention prettier-regression-testing
* Fix the type for snippets
* Typo
* Add 'Advanced topics' heading
* Fix run_spec types
* Minor edits
* Add link
* Wording around yarn fix
* Update CONTRIBUTING.md
Co-authored-by: fisker Cheung <lionkay@gmail.com>
