[air/docs] Add example to fetch results dataframe for trainer/tuner

[krfricke]

Signed-off-by: Kai Fricke [email protected]

Why are these changes needed?

Our docs examples should contain example code to fetch dataframes from result/result grid objects

Related issue number

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• 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 :(