Redirect published security-built-in-authentication URL to new diataxis page

This is a required sub-task of https://issues.redhat.com/browse/QDOCS-88.

Because we are retiring the legacy topic `security-built-in-authentication.adoc`, we need to redirect bookmarks and other doc links to the best new target content, which in this sprint is security-authentication-mechanisms-concept.

The original URL https://quarkus.io/guides/security-built-in-authentication needs to be redirected to https://quarkus.io/guides/security-authentication-mechanisms-concept.

This task is done when:

• A redirect markdown config file is created in the redirects folder of the `Develop` branch in the quarkusio.github.io website portal repo: https://github.com/quarkusio/quarkusio.github.io as outlined in the following resources:

1. https://quarkus.io/version/main/guides/doc-contribute-docs-howto#retire-and-redirect-an-existing-quarkus-asciidoc-source-file
2. Demo : https://drive.google.com/file/d/1v_XliNIIEmBV5L_Fr2gsIpOhhsktpy0V/view?usp=sharing