Add definition of GVR in docs

[dannysauer]

While #1175 added a definition of GVK, it did not add a definition of GVR. Thus, #1142 was closed as complete when only half complete. I can't reopen the ticket, so opening a new one. :)

Hello @chipzoller thank you for addressing this. I have looked at the PR and can't find the actual definition of the acronyms. Would you agree, that this is still missing? Kind regards Alexander.

Originally posted by @aschaber1 in #1142

CC: @chipzoller