Documentation: document process for creating an ISIS areadetector IOC

As a developer I would like documentation so that I can create an ISIS-style area detector IOC.

This has come up as part of the review of https://github.com/ISISComputingGroup/IBEX/issues/4039 . We have documentation for other more advanced epics features e.g. [aSub records](https://github.com/ISISComputingGroup/ibex_developers_manual/wiki/aSub-records), [SNL](https://github.com/ISISComputingGroup/ibex_developers_manual/wiki/Creating-a-State-Machine-(Sequencer)), it would be good to add an equivalent of those pages for areadetector.