Write docs on DCMs

[DominicOram]

As part of trying to standardise DCMs some docs should be written about how DCMs are handled in dodal and what the minimum expected behavior is from the EPICS side

Acceptance Criteria

• Docs are written on what a minimum DCM looks like (@DominicOram)
• There are also docs from the controls perspective on how to build one in a container (@coretl)

[DominicOram]

It looks like beamlines also use pitch and roll so these should be included in the external interface