Restore intersphinx in docs config

[tacaswell]

#21 removed the interphinx extension because it was failing with miss-configured external references. The task is

1. restore using intersphinx
2. fix the URLs for any inventories we need

[mrakitin]

Is it because the event-model docs were moved to its repo's branch https://github.com/bluesky/event-model/tree/gh-pages?