feat(python-library): changes to docs job

* feat(python-library): changes to docs job

* migrate to Trampoline V2
* add docs-presubmit job
* create docfx yaml files and upload them to another bucket

* remove redundant envvars

* add a failing test first

* fix TemplateSyntaxError: Missing end of comment tag

* serving_path is not needed any more

* use `raw` to make jinja happy

Source-Author: Takashi Matsuo <tmatsuo@google.com>
Source-Date: Thu Jul 30 12:44:02 2020 -0700
Source-Repo: googleapis/synthtool
Source-Sha: 5dfda5621df45b71b6e88544ebbb53b1a8c90214
Source-Link: googleapis/synthtool@5dfda56