Add "Edit on Github" links for readthedocs (#246)

As explained in https://docs.readthedocs.io/en/stable/guides/edit-source-links-sphinx.html

Author: aabmass

docs/conf.py

@@ -99,3 +99,11 @@
     "sidebar_width": "320px",
     "body_max_width": "800px",
 }
+
+html_context = {
+    "display_github": True, # Integrate GitHub
+    "github_user": "GoogleCloudPlatform", # Username
+    "github_repo": "opentelemetry-operations-python", # Repo name
+    "github_version": "main", # Version
+    "conf_py_path": "/docs/", # Path in the checkout to the docs root
+}