Add Edit On Github button

This button should show up at the top-right on the Read The Docs
template.

This fixes the problem with the current button showing "View page
source" which displays the raw .rst file backing up the page.

docs/conf.py

@@ -15,3 +15,10 @@
 
 extensions = ["sphinx_rtd_theme"]
 html_theme = "sphinx_rtd_theme"
+
+html_context = {
+    "display_github": True,
+    "github_user": "kraken-tech",
+    "github_repo": "django-pg-migration-tools",
+    "github_version": "main/docs/",
+}