Document how to use topology aware hints with Linkerd

[mateiidavid]

Linkerd used to support topology keys for services. The feature never made it out of alpha in the Kubernetes API, and it was replaced with topology aware hints on endpoints.

The Linkerd team deprecated topology keys in favour of topology hints (see PR linkerd2/#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.