Comprehensive limits documentation

[drok]

Refer to "System limits" documentation, https://etcd.io/docs/v3.5/dev-guide/limit/

This page currently includes two (2) limits; the individual request size limit and the total disk usage limit.

It should include limits one would be interested to know, like:

• Maximum number of keys, directories, keys per directory
• Maximum number of reads / writes per second (there seem to be various stability concerns like OOM and leader re-elections)
• Formula tying various limits together; I've seen something mentioned in benchmarks, about how RAM usage relates to number of keys/clients/watchers

A developer looking for limits guidance would want to be able to map his envisioned application to the limits of etcd, and the current two limits are not helpful.

Thanks