Document how to use topology aware hints with Linkerd

Linkerd used to support [topology keys for services](https://linkerd.io/2020/11/23/topology-aware-service-routing-on-kubernetes-with-linkerd/). The feature never made it out of alpha in the Kubernetes API, and it was replaced with [topology aware hints](https://kubernetes.io/docs/concepts/services-networking/topology-aware-hints/) on endpoints.

The Linkerd team deprecated topology keys in favour of topology hints (see [PR linkerd2/#6698](https://github.com/linkerd/linkerd2/pull/6698)). Both topology keys and hints went undocumented. We should perhaps add at least a paragraph where we can tell users how to turn the feature on, and where we can link to additional reading (such as the Kubernetes docs).

**Note**: stable-2.12.x should come with endpointslices turned on by default, a prerequisite for using this feature. A note/warning should also be included to inform users that the feature gate **MUST** be turned on in their cluster.

Use the existing Linkerd website documentation as the entry point, then read the linked Kubernetes topology-aware hints documentation and linkerd2/#6698. Add guidance on enabling topology-aware hints, link to further reading, and warn that the cluster feature gate must be enabled; account for EndpointSlices being enabled by default in stable-2.12.x.

Written by the indexing model from the issue text.