Add section to coding guidelines on Optional usage (#6894)

This was discussed a long while back, but never documented (and recently
came up in #6889)

Co-authored-by: Fabrizio Ferri-Benedetti <fferribenedetti@splunk.com>

docs/contributing/style-guideline.md

@@ -126,3 +126,11 @@ plugins {
   id("otel.nullaway-conventions")
 }
 ```
+
+## java.util.Optional usage
+
+Following the reasoning from [Writing a Java library with better experience (slide 12)](https://speakerdeck.com/trustin/writing-a-java-library-with-better-experience?slide=12),
+usage of `java.util.Optional` is kept at a minimum in this project.
+
+It is ok to use `Optional` in places where it does not leak into public API signatures and where
+performance is not critical.