Add Valkey to onprem in-memory databases

Feature request

Add a Valkey node class to diagrams.onprem.inmemory, alongside the existing Redis class.

Motivation

Valkey is an open source, high-performance key/value datastore that was forked from Redis after Redis changed its licensing. It is stewarded by the Linux Foundation and has seen broad adoption (AWS ElastiCache, Google Cloud Memorystore, and others now offer Valkey). Many on-prem and cloud architectures now use Valkey in place of Redis, so it would be useful to represent it in diagrams.

Proposed change

• Add a Valkey class in diagrams/onprem/inmemory.py:
  python复制

  class Valkey(_Inmemory):
      _icon = "valkey.png"

• Add the valkey.png icon resource under resources/onprem/inmemory/.
• Document the new node in docs/nodes/onprem.md.

Notes

An open PR already implements this: #1230.