Replace use of BEAMLINE with INSTRUMENT

As part of integrating with the super graph, groups need to standardise on names for commonly shared ideas (e.g. for LIMS a "session" is what DAQ call a "visit" is: so a compromise position of "instrument session" has been reached, meaning we can both write code that deals with "a specific allocated time slot on a specific allocated set of hardware" and it can interoperate.
An "instrument" is such a set of hardware: beamlines, microscopes, labs, etc. - we already have p38 (lab), p45-p49 (test rigs) and the adsim (simulated hardware) defined in dodal, and @dan-fernandes is looking to add the Optics lab to that number.

Proposals (not all equal in importance or urgency):
- Rename get_beamline_name to get_instrument_name and allow it to read INSTRUMENT environment variable instead/as well
- Rename the dodal.beamlines module to dodal.instruments (this would require a major version bump)
- Rename the dodal.beamlines.beamline_utils module
- Rename BeamlinePrefix
- Move the code currently in dodal.beamlines.specific_beamline_utils into dodal.instruments.i03
- Rename and modify the contents of [create-beamline.rst](https://github.com/DiamondLightSource/dodal/blob/5893821fb40e3bc334bea7c172de34d36d0d2b2c/docs/how-to/create-beamline.rst#L7)
- Find and replace other places beamline is used as a term in the generic, and add documentation explaining that dodal supports all forms of instrument of which beamlines are just one type

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