Make "Edit on GitHub" always point to master branch

Fixes #1138.

conf.py

@@ -66,6 +66,15 @@ html_theme_options = {
     # 'navigation_depth': 4,  # Depth of the headers shown in the navigation bar
 }
 
+# VCS options: https://docs.readthedocs.io/en/latest/vcs.html#github
+html_context = {
+    "display_github": True, # Integrate GitHub
+    "github_user": "godotengine", # Username
+    "github_repo": "godot-docs", # Repo name
+    "github_version": "master", # Version
+    "conf_py_path": "/", # Path in the checkout to the docs root
+}
+
 html_logo = 'img/docs_logo.png'
 
 # Output file base name for HTML help builder