Example device creation process and write tests docs

[oliwenmandiamond]

Fixes #634

Instructions to reviewer on how to test:

1. Check tests pass
2. Check added documentation makes sense and relevant.

Checks for reviewer

• Would the PR title make sense to a scientist on a set of release notes
• If a new device has been added does it follow the standards
• If changing the API for a pre-existing device, ensure that any beamlines using this device have updated their Bluesky plans accordingly
• Have the connection tests for the relevant beamline(s) been run via dodal connect ${BEAMLINE}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.75%. Comparing base (f3b53e9) to head (7595aed).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1484   +/-   ##
=======================================
  Coverage   98.75%   98.75%           
=======================================
  Files         235      235           
  Lines        8645     8645           
=======================================
  Hits         8537     8537           
  Misses        108      108           

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[oliwenmandiamond]

I have updated the docs on how to test your enums match epics and what you need to know when writing tests on devices in dodal. I am not sure my example is the best, so if you're able to suggest a better example for me to use that would be great! Thanks

[noemifrisina]

Just a couple of comments, it's good overall, just a bit clunky in a couple of places.
Example is nice, I don't think we need anything else to give a good idea what to do.