[Dashboard][Hotfix] Update dashboard build instructions for new dashboard

[mfitton]

Why are these changes needed?

The old dashboard build instructions in building from source do not work for the new dashboard, this fixes that issue.

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 :(

[simon-mo]

Can you also remove these in git so it won't be problem for people building from source in the future?
deleted: dashboard/client/build/.fakebuild
deleted: dashboard/client/build/static/.fakestatic