Dynamic navbar: navbar item activation strategies

## 🚀 Feature

An example:

I have three instances of docs, each segmented into three categories with their respective sidebars: "concepts" "guides" and "reference". These docs can be called "V1" "V2" and "V3", but are fixed docs once released so versioning isn't helpful in the canonical sense. When on "V2"-"Concepts" - clicking the Guides button on the navbar will take me to "V2"-"Guides", along with the appropriate sidebar.

### Have you read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/master/CONTRIBUTING.md#reporting-new-issues)?

yes

## Motivation

I'm trying to make the above work and have been unsuccessful after many days of wrangling

## Pitch

It seems useful to have navbar routing that can accommodate multiple doc instances