[Docs] Write up developer documentation

[mmcky]

Description / Summary

Need to write up lessons learnt with integrating custom admonitions to support:

1. integration with ref and numref
2. setting up directives
3. working with post_transforms
4. working with transforms

Value / benefit

This will help with some more general docs for writing extensions for sphinx that could be of value to the executable books community

Implementation details

No response

Tasks to complete

No response