chore: add infrastructure to support `docs-presubmit` build (via synth) (#578)

* 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: https://github.com/googleapis/synthtool/commit/5dfda5621df45b71b6e88544ebbb53b1a8c90214

* fix(python-library): add missing changes

Source-Author: Takashi Matsuo <tmatsuo@google.com>
Source-Date: Thu Jul 30 18:26:35 2020 -0700
Source-Repo: googleapis/synthtool
Source-Sha: 39b527a39f5cd56d4882b3874fc08eed4756cebe
Source-Link: https://github.com/googleapis/synthtool/commit/39b527a39f5cd56d4882b3874fc08eed4756cebe

Co-authored-by: Tres Seaver <tseaver@palladion.com>
diff --git a/synth.metadata b/synth.metadata
index 901a2cb..2563871 100644
--- a/synth.metadata
+++ b/synth.metadata
@@ -4,14 +4,14 @@
       "git": {
         "name": ".",
         "remote": "https://github.com/googleapis/google-auth-library-python.git",
-        "sha": "218a159f646c81021c890b92f9cff003aed949a8"
+        "sha": "20f82e22b7e8c6c7fdd29e08eaf7b4cf2abdcf37"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "ffe10407ee2f261c799fb0d01bf32a8abc67ed1e"
+        "sha": "39b527a39f5cd56d4882b3874fc08eed4756cebe"
       }
     }
   ]