[security.adoc] Create concept adoc for authentication mechanisms in Quarkus content

This task is done when:

• New diataxis template concept topic is created for Authentication Mechanisms in Quarkus.
• Existing content identified on the Miro board is moved into this concept topic.
• Minimal style effort.
• Xrefs and links are updated accordingly throughout the other topics in the repo.
• All is SME approved and merged to main.

Original state before poc: https://quarkus.io/version/2.7/guides/#security