feat(userprefs): update documentation and list extensions endpoint

[laurentiuNiculae]

What type of PR is this?

Which issue does this PR fix:
Closes #1455

What does this PR do / Why do we need it:

If an issue # is not available please add repro steps and logs showing the issue:

Testing done on this change:

Automation added to e2e:

Will this break upgrades or downgrades?

Does this PR introduce any user-facing change?:

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@970997f). Click here to learn what that means.
The diff coverage is 100.00%.

❗ Current head 2932ccd differs from pull request most recent head 3dee6d0. Consider uploading reports for the commit 3dee6d0 to get more accurate results

@@           Coverage Diff           @@
##             main    #1456   +/-   ##
=======================================
  Coverage        ?   90.92%           
=======================================
  Files           ?      103           
  Lines           ?    22608           
  Branches        ?        0           
=======================================
  Hits            ?    20557           
  Misses          ?     1528           
  Partials        ?      523           

Impacted Files	Coverage Δ
pkg/extensions/extension_mgmt.go	90.47% <100.00%> (ø)
pkg/extensions/extension_mgmt_disabled.go	100.00% <100.00%> (ø)
pkg/extensions/extension_search.go	97.27% <100.00%> (ø)
pkg/extensions/extension_search_disabled.go	100.00% <100.00%> (ø)
pkg/extensions/extension_userprefs.go	91.42% <100.00%> (ø)
pkg/extensions/extension_userprefs_disable.go	100.00% <100.00%> (ø)
pkg/extensions/get_extensions.go	100.00% <100.00%> (ø)

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