📝(doc) add Federated Nexus docs instance (#1320)

## Purpose

Adds a new Docs instance, as suggested here:
https://github.com/suitenumerique/docs/discussions/1316

## External contributions

Thank you for your contribution! 🎉  

Please ensure the following items are checked before submitting your
pull request:
- [x] I have read and followed the [contributing
guidelines](https://github.com/suitenumerique/docs/blob/main/CONTRIBUTING.md)
- [x] I have read and agreed to the [Code of
Conduct](https://github.com/suitenumerique/docs/blob/main/CODE_OF_CONDUCT.md)
- [x] I have signed off my commits with `git commit --signoff` (DCO
compliance)
- [x] I have signed my commits with my SSH or GPG key (`git commit -S`)
- [x] My commit messages follow the required format: `<gitmoji>(type)
title description`
- [ ] I have added a changelog entry under `## [Unreleased]` section (if
noticeable change)
- [ ] I have added corresponding tests for new features or bug fixes (if
applicable)

Signed-off-by: Henry-Hiles <henry@henryhiles.com>

README.md

@@ -63,6 +63,7 @@ We hope to see many more, here is an incomplete list of public Docs instances (u
 | docs.suite.anct.gouv.fr    | ANCT    | French public agents working for the territorial administration and the extended public sphere. ProConnect is required to login in or sign up|
 | notes.demo.opendesk.eu    | ZenDiS    | Demo instance of OpenDesk. Request access to get credentials |
 | notes.liiib.re    | lasuite.coop    | Free and open demo to all. Content and accounts are reset after one month |
+| docs.federated.nexus    | federated.nexus    | Public instance, but you have to [sign up for a Federated Nexus account](https://federated.nexus/register/). |
 
 #### ⚠️ Advanced features
 For some advanced features (ex: Export as PDF) Docs relies on XL packages from BlockNote. These are licenced under GPL and are not MIT compatible. You can perfectly use Docs without these packages by setting the environment variable `PUBLISH_AS_MIT` to true. That way you'll build an image of the application without the features that are not MIT compatible. Read the [environment variables documentation](/docs/env.md) for more information.