[Docs] Update code-tutorials for 8.4

Several minor docs updates:

• update urls that referenced the Developer guide that was removed in 8.4
• Update repository url to point to 8.4.0
• mention that Java 11 (at least) is a requirement now
• fix broken link to the operator doc
• change the attribute for the operator docs to make it consistent with the rest of the docs
• fix link to the Infinispan Wildfly modules that were moved from Dev guide to the Embedded guide