docs(release): v0.2.3 notes cover the shared descriptors

[roncodes]

RELEASE.md only mentioned the customer places fix. The notes now also cover the shared resource identity descriptors and wrappers (#86), the identity stub builder and type label fixes (#87), and opening records on their Fleet-Ops page from other engines (#88), with an upgrading note on the ember-ui and fleetops versions they pair with.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (d728efb) to head (550c866).

Additional details and impacted files

@@               Coverage Diff                @@
##           release/v0.2.3       #89   +/-   ##
================================================
  Coverage          100.00%   100.00%           
================================================
  Files                 121       121           
  Lines                3157      3157           
  Branches              777       777           
================================================
  Hits                 3157      3157           

Flag	Coverage Δ
fleetops-data	100.00% <ø> (ø)

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