Complete CHANGELOG for r2.3 release - #134

[P-022] release-plan.yaml was changed alongside 16 other file(s)%3A CHANGELOG/CHANGELOG-r2.md, README.md, code/API_definitions/network-slice-assignment.yaml, code/API_definitions/network-slice-booking.yaml, code/Test_definitions/network-slice-assignment-assignDevice.feature, code/Test_definitions/network-slice-assignment-getDevices.feature, code/Test_definitions/network-slice-assignment-releaseDevice.feature, code/Test_definitions/network-slice-assignment-retrieveSlicesByDevice.feature, code/Test_definitions/network-slice-booking-createSlice.feature, code/Test_definitions/network-slice-booking-deleteSlice.feature, and 6 more. release-plan.yaml changes should be submitted in a dedicated PR so that any new validation findings remain clearly attributable to the release-plan change. | Suggestion%3A Split this PR%3A keep release-plan.yaml changes in a dedicated PR, and move the other listed files to a separate PR.