38309: docs: add an Insights into Constraint Conformance RFC r=andreimatei a=andreimatei

The set of features described here aim to provide admins with visibility into
aspects of replication and replica placement. In particular admins will be able
to access a report that details which replication constraints are violated.

Release justification: non-code change
Release note: None

76909: settings: disallow tenant from changing overridden settings r=ajwerner a=RaduBerinde

Informs #73857.

#### settings: simplify default overrides

This commit cleans up the Override() implementation, using a
single `map[slotIdx]interface{}`.

Release note: None

#### settings: disallow tenant from changing overridden settings

We now produce an error if a tenant tries to change a cluster setting
that is currently overridden by the operator. The previous behavior
was too subtle: we would change the tenant's "opinion" on the setting,
which would only take effect if/when the operator override was
removed.

To achieve this we query the settings watcher which always has the
current list of overrides. In the future, we should somehow integrate
this information in `settings.Values`.

Release justification: bug fixes and low-risk updates to new functionality

Release note: None

Co-authored-by: Andrei Matei <[email protected]>
Co-authored-by: Radu Berinde <[email protected]>