Update list of frontend examples in README
Update list of frontend examples in README

This list contained an example that is no longer rendered by Whitehall,
so that has been removed. Also adding examples of all other document
types rendered by Whitehall, so there are examples of all currently
rendered documents.

Updates can be made when rendering is moved away, until the list is
fully removed.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall