Add central place to define version numbers + support for variables in markdown

[greenbreakfast]

The Idea

Since version numbers (like OpenWRT 23.05.3, and NodeJS v16.19) change fairly often and are included in several articles, it would be nice to:

1. Have a central place to define all of these values (like a JSON file or similar)
2. Enable support for variables in markdown rendering
3. Update articles to use variables instead of hard-coding version numbers

That way, when a version number changes, only one file has to be manually updated and the rendered documentation site will be updated in all places the version number is used.

Execution

Need more research to figure out how to do this

[greenbreakfast]

Implementation Research

Use preprocessor suggestion from facebook/docusaurus#395 (comment)
Tested with existing 2.4.3 Docusaurus, didn't work

Requires upgrade to Docusaurus 3.x