docs: Add RTD local_settings file, to add custom templates.

docs/readthedocs/settings/local_settings.py

+import os
+
+# Directory that the project lives in, aka ../..
+SITE_ROOT = '/'.join(os.path.dirname(__file__).split('/')[0:-2])
+
+TEMPLATE_DIRS = (
+    "%s/templates/" % SITE_ROOT, # Your custom template directory, before the RTD one to override it.
+    "%s/readthedocs/templates/" % SITE_ROOT, # Default RTD template dir
+)