Rate limiting Redis backend

[happyRip]

Summary

Document new configuration options of rate limiting provider.

Why do we need this ?

After finishing the work on https://github.com/TheThingsIndustries/lorawan-stack/issues/2969 we need to document the new features.

What is already there? What do you see now?

Rate limiting docs.

What is missing? What do you want to see?

Redis rate limiting docs.

How do you propose to document this?

Add missing docs.

Can you do this yourself and submit a Pull Request?

Yes

[KrishnaIyer]

@happyRip: Can you please add this documentation for the new release?