Docs versioning

[agriffard]

Add a version selector dropdown to the documentation site in order to switch between different documentation versions of the releases.
Docs site is located here: https://docs.orchardcore.net/en/latest/releases/2.2.1/
It is using MkDocs and a Material theme (https://github.com/squidfunk/mkdocs-material, https://squidfunk.github.io/mkdocs-material/)
The contents used for documentation are in .md files located in src/docs folder.

Releases started with version 1.0.
Current release is 2.2.1 (Changelog is here: https://docs.orchardcore.net/en/latest/releases/2.2.1/) so the current version of docs should display "2.2" (major.minor)
Next version will be 3.0 (latest from main).

One solution could be to use this feature: https://squidfunk.github.io/mkdocs-material/setup/setting-up-versioning/

Explain how adding the version would impact the urls of the documentation pages.
Document the procedure and the required steps to execute when a new release is created and we expect to obtain a new version in the documentation site.
Take screenshots of the result to check that the version selector is correctly displayed in the header of the documentation site.

[github-actions]

We triaged this issue and set the milestone according to the priority we think is appropriate (see the docs on how we triage and prioritize issues).

This indicates when the core team may start working on it. However, if you'd like to contribute, we'd warmly welcome you to do that anytime. See our guide on contributions here.