Add NHS login to patterns

[anandamaryon1]

We occasionally get queries about how to use NHS Login, with users expecting to find info about it in the service manual, when in fact it has its own guidance.

We could add NHS login as a pattern, and use it to link to the guidance.

We are also going to be adding the NHS Login button as a component, which will link out to the guidance too, so perhaps we can publish the component and monitor to see whether a pattern would be helpful in addition.