[security-getting-started.adoc & security-builtin-authentication.adoc] Finish the concept topic for Basic authentication in Quarkus

This task is done when:

• A new diataxis concept topic is created that Basic authentication in Quarkus.
• Existing content in security-builtin-authentication.adoc and security-getting-started.adoc is migrated into the new topic before the legacy adoc is retired.
• 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