guides/integrations/memcached: document Java wrapper

[odeke-em]

I recently released the source code to a SpyMemcached wrapper at https://github.com/orijtech/ocspymemcached and importable by

<dependency>
  <groupId>io.orijtech.integrations</groupId>
  <artifactId>ocspymemcached</artifactId>
  <version>0.0.1</version>
</dependency>

This issue is a placeholder to create a guide on the website with a complete copy+pastable end-to-end example and screenshots of observability signals.