[v3.0] Improve user-extensibility of generated UVM code

[amykyta3]

Currently, there isn't really a well-defined mechanism to augment the generated UVM code.

Refactor the template scheme to provide "official" mechanisms that allow users to augment the generated UVM.
This includes:

• Overrides to base classes
• Template hooks to allow insertion of user-defined code in all the major UVM register model methods.
• Template hooks for inserting additional user methods & class variables

All of this to be properly documented in a readthedocs.io page.