Consider using a /.well-known/ URI for service discovery

[alexdutton]

This would probably be a redirect from e.g. /.well-known/sword-v3 to the service document.

Once in the spec, it should be submitted to the Well-Known URI registry

[richard-jones]

• [x] add a section to the Auto Discovery section for well-known

• [x] add a reference to the well-known rfc

[richard-jones]

Keeping this open to track the need for us to register this with Well-Known once the final version is released