docs: update dev environment instructions

[crisbeto]

Updates the dev environment instructions to mention how to run tests properly.

Also removes global Gulp as a required step for setting up the environment and changes the Gulp commands to go through yarn.

[devversion]

LGTM.

[crisbeto]

I've addressed the feedback.

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
     status "ci/circleci: e2e_tests" is failing
     status "ci/circleci: tests_saucelabs" is pending

If you want your PR to be merged, it has to pass all the CI checks.

If you can't get the PR to a green state due to flakes or broken master, please try rebasing to master and/or restarting the CI job. If that fails and you believe that the issue is not due to your change, please contact the caretaker and ask for help.

[Splaktar]

LGTM

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