Document display driver/panel enum list location and keep it maintained

Source: [brentru comment on line 214](https://github.com/adafruit/Wippersnapper_Protobuf/pull/170#discussion_r2990315560) (UNRESOLVED)

"Are these kept somewhere, like in a repo? Can we point or reference that location from here? This list will grow"

Suggested title: Document and reference the canonical display driver/panel enum list Suggested body:

From PR #170 review: The display driver and panel enums in display.proto will grow over time as new displays are supported. We need to:

Establish a canonical location where these enums are maintained/documented
Add a comment in display.proto pointing to that reference location
Consider a process for keeping the list updated as new drivers are added
Ref: https://github.com/adafruit/Wippersnapper_Protobuf/pull/170#discussion_r2990315560