Revise BELL custom user registry example

[dmuelle]

Per review in #5724 , the custom user registry example on the BELL feature page needs to be revised and tested.

The is example as presently published will be removed from the doc with 22.0.0.11- it isn't helpful in the current state. I'll post the example as is to this issue for reference.

[dmuelle]

@dazavala comments on this example from #5724

THIS SECTION SHOULD BE IMPROVED. I'M NOT YET COMFORTABLE REVISING THIS SECTION W/O INVESTIGATING THE SECURITY FEATURE AND PROVING AN EXAMPLE. THERE'S NO EXAMPLE HERE NOW, OR AT LEAST NO DESCRIPTION OF THE LIBRARY COMPOSITION.

FOUND RECENT (2021) SUPPORT DOCS+BLOGS ON THIS TOPIC. THE EXAMPLES SEEM WRONG IN THAT THEY DO NOT ENABLE ANY SECURITY FEATURE THAT EXPORTS THE API PACKAGE -- THE UserRegistry INTERFACE -- REQUIRED BY THE SERVICE IMPLEMENTATION CLASS PACKAGED IN THE BELL LIBRARY. NEEDS INVESTIGATION. ALSO, SECTION DOES NOT MENTION THE INTERFACE OR CONTEXTUAL INFO, FOR EXAMPLE: "You can develop a custom user registry by implementing the com.ibm.websphere.security.UserRegistry interface. etc."

REF: https://www.ibm.com/support/pages/how-create-custom-user-registry-websphere-liberty

REF: https://github.com/OpenLiberty/docs/issues/2418

REF: https://community.ibm.com/community/user/wasdevops/blogs/hiroko-takamiya1/2021/11/19/configuring-custom-user-registry-liberty?CommunityKey=5c4ba155-561a-4794-9883-bb0c6164e14e&tab=recentcommunityblogsdashboard

[dmuelle]

We also have this previous issue on BELL/CUR that has a good bit of discussion from OL users, but has been iceboxed for over a year

https://github.com/OpenLiberty/docs/issues/2418