docs: add 'edit-on-github' link to html theme

Closes IDF-1504

docs/get_github_rev.py

@@ -0,0 +1,15 @@
+import subprocess
+
+
+# Get revision used for constructing github URLs
+def get_github_rev():
+    path = subprocess.check_output(['git', 'rev-parse', '--short', 'HEAD']).strip().decode('utf-8')
+    try:
+        tag = subprocess.check_output(['git', 'describe', '--exact-match']).strip().decode('utf-8')
+    except subprocess.CalledProcessError:
+        tag = None
+    print('Git commit ID: ', path)
+    if tag:
+        print('Git tag: ', tag)
+        return tag
+    return path