chore(py_library): add some excludes in docs/conf.py

This should fix build failures in python-bigquery.
Example:
googleapis/python-bigquery#205

* also add a comment

Source-Author: Takashi Matsuo <tmatsuo@google.com>
Source-Date: Mon Aug 3 15:08:00 2020 -0700
Source-Repo: googleapis/synthtool
Source-Sha: 4f8f5dc24af79694887385015294e4dbb214c352
Source-Link: googleapis/synthtool@4f8f5dc

docs/conf.py

@@ -21,6 +21,7 @@
 sys.path.insert(0, os.path.abspath(".."))
 
 # For plugins that can not read conf.py.
+# See also: https://github.com/docascode/sphinx-docfx-yaml/issues/85
 sys.path.insert(0, os.path.abspath("."))
 
 __version__ = ""
@@ -93,7 +94,12 @@
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.
-exclude_patterns = ["_build"]
+exclude_patterns = [
+    "_build",
+    "samples/AUTHORING_GUIDE.md",
+    "samples/CONTRIBUTING.md",
+    "samples/snippets/README.rst",
+]
 
 # The reST default role (used for this markup: `text`) to use for all
 # documents.

synth.metadata

@@ -19,14 +19,14 @@
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "bfcdbe0da977b2de6c1c0471bb6dc2f1e13bf669"
+        "sha": "4f8f5dc24af79694887385015294e4dbb214c352"
       }
     },
     {
       "git": {
         "name": "synthtool",
         "remote": "https://github.com/googleapis/synthtool.git",
-        "sha": "bfcdbe0da977b2de6c1c0471bb6dc2f1e13bf669"
+        "sha": "4f8f5dc24af79694887385015294e4dbb214c352"
       }
     }
   ],