Documentation on how to use extensions (as a user)

[holly-cummins]

Description

We have lots of documentation on how to write extensions, but less on how to use them, and nothing that's neatly gathered up in one place. I'm looking for something I can link to as "What are extensions? How to use extensions?".

Implementation ideas

The page at https://access.redhat.com/documentation/en-us/red_hat_build_of_quarkus/1.3/html/developing_and_compiling_your_quarkus_applications_with_apache_maven/proc-installing-and-managing-java-extensions-with-quarkus-applications_quarkus-maven#doc-wrapper would be perfect, except it's Red Hat-specific, rather than community documentation. Also relevant is https://quarkus.io/faq/#what-is-a-quarkus-extension, but it's only a paragraph mixed in with a longer set of content.

[quarkus-bot[bot]]

/cc @MichalMaler, @ebullient, @hmanwani-rh, @inoxx03, @michelle-purcell, @sheilamjones, @sunayna15

[holly-cummins]

Once this is done, we should update https://github.com/quarkusio/extensions.quarkus.io/blob/main/src/components/navigation.js and the corresponding menus in quarkus.io.