docs: fix missing styles on doc error pages (#573)

This PR attempts to fix unstyled error pages
([example](https://documentation.ubuntu.com/pebble/asdf)) by setting the
Sphinx project slug. I can't test this locally, so we'll need to merge
and see what happens.

docs/custom_conf.py

@@ -115,7 +115,7 @@
 
 # If your project is on documentation.ubuntu.com, specify the project
 # slug (for example, "lxd") here.
-slug = ""
+slug = "pebble"
 
 ############################################################
 ### Redirects