Add documentation for redis & drop-in replacements

[waldbauer-certat]

As redis is a nice in-memory database and field-tested, there are some competitors out there working on more advanced solutions to solve issues redis is currently having.

Some of them are:

• https://docs.keydb.dev/ ( Also field-tested, faster processing because of its internal architecture (multi-threaded) )
• https://github.com/ideawu/ssdb ( Not tested yet, but is built from scratch )
• https://github.com/cbd/edis

Why should we add more options than redis? Well, mostly because every workflow differs, so adding the multiple choices for different workflows are a nice to have and improve the overall quality and usability.