Expand RRC usage and handover documentation to describe the two-level
neighbour configuration layout, lookup-key semantics,
and config-time SIB4 grouping behaviour. Expand the example config
to include a `frequency_list` block and per-neighbour SIB3/SIB4
offset fields. Add SIB2 config example.

Update handover-tutorial.md to describe the same nested model and
note that F1 and N2 handover share the same serving-cell keyed
mapping.

Add documentation about SIB3/SIB4 and MeasGaps implementation
in OAI with stress on the shared neighbour configuration data model.

Update FEATURE_SET.