fix: some documentation fixes for bazel gcp (#3505)

* fix: some documentation fixes for bazel gcp

Update old docs that were about previous gcp bazel extractor, instead
describe kythe/extractors/bazel docker image.

Modify `extract.sh` to use `$KYTHE_OUTPUT_DIRECTORY` and pass it in
accordingly, as a suggestion at the end of #3499.  (I'm not sold on this
either way, this is just a suggestion)
3 files changed