Document pre-submit procedure.

As a new contributor, it is not clear what steps should be done to test a change locally before sending a PR. Some ideas of information to include based on issues I've encountered:

- Which tests to run and the respective `bazel` commands to run them.
- Info on updating docs (CI will let you know about `./docs/update_docs.sh` but relying on that adds latency to the PR process)
- Running `buildifier`. (It's unclear from a failed CI run how to invoke `buildifier` with the same options that are used in CI. CI does not print out the command here. Failures here also add latency to the PR process).