[Doc] Improve development instructions

[mwtian]

Why are these changes needed?

Clarify the development instructions to

• remove explicit install of cython and pytest, which are installed inside setup.py.
• add a section on installing test dependencies.

Also, reword a button in the dashboard.

Related issue number

Checks

• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(