New custom container for "Sources" boxes

See how the "upstream source" is reported in this articles:

- https://gbdev.io/pandocs/IR.html#gbc-infrared-communication
- https://gbdev.io/pandocs/Gameboy_Camera.html

I suggest preparing a new "custom container" to use in place of info boxes. Smaller font, maybe the highlight with a non-accent color (gray?).

We should also think about specifying (consistently) if the source should be considered the upstream against which contribute (e.g. Dan Docs you should contribute upstream, but the for the GB Camera document the author expressed the idea to just import it in Pan Docs and keep working on it there)

Start by comparing the upstream-source sections in the linked IR and Game Boy Camera articles. Determine how a custom container would replace the current info boxes and how the upstream contribution guidance should be stated consistently. Done means the container styling and source guidance are defined for both cases.

Written by the indexing model from the issue text.