Update device factory docs

[DiamondJoseph]

Updates docs on how to create a beamline for changes in ophyd-async and dodal.
After #405 goes in with the epics-containers mocks, we could have the docs built against those devices?

Instructions to reviewer on how to test:

1. Read the changed docs to ensure they make sense
2. Make the example beamline described in the docs
3. Ensure that devices connect/skip/mock appropriately

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 95.19%. Comparing base (ecc1a20) to head (7d1d0cb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #855   +/-   ##
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  Coverage   95.19%   95.19%           
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  Files         120      120           
  Lines        4976     4976           
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  Hits         4737     4737           
  Misses        239      239           

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