More info for users about councils

[pmk01]

I'd like to have a bit of info for users about what councils are & what they do. We usually put bespoke explainers on all election types except councils. At the moment this is done manually, and would be a chore to add explainers to all council elections.

Ideally this would be unique to council type. Something like:

Bath and North East Somerset Council is a Unitary Authority. Learn more about councils in the UK.

Other info could be added - what each council type does, for example. But this is a simple starter.

(Suggestions welcomed for alternative link. Maybe: https://www.electoralcommission.org.uk/i-am-a/voter/types-elections/local-councils)

This issue is probably solved in the same way as #1297