Additional "Class Reference" sections to docs please

I regenerate the API docs whenever there appears to be a change affecting them, and no later than with each release.

All three classes are there, for class reference:

Validation ... https://codeigniter4.github.io/api/class...ation.html
Model ... https://codeigniter4.github.io/api/class...Model.html
Entity ... https://codeigniter4.github.io/api/class...ntity.html

Part of my plan is to have cross-links between the user guide and the API docs, but that hasn't happened yet.