RFC: context-aware version dropdown for multi-instance setups

[Zenahr]

************************************* DRAFT *************************************

Motivation

This RFC is a natural evolution from discussions on Discord with me, @slorber and @homotechsual.
See: https://discordapp.com/channels/398180168688074762/867060369087922187/1060875802801098793 to follow the prior discussion.

Use-case scenario

• You have several products to document, each with their own release cycle.
• You'd like to synchronize the docs to each product cycle.

Current solution and problems

To illustrate the problem with just using multi-instance versioning out-of-the-box:

... (to be continued.)

[Zenahr]

I'll keep working on this in the following weeks/months. I hope it's fine for me to use this as a draft space and jot down my thoughts until I've formulated a complete, potential design solution.

[slorber]

ok for me

seems related to #4389