Add clustering sample with ClusteringProvider - #3342

Adds a new ClusteringSample demonstrating grid-based feature clustering on a map.

Changes:

• ClusteringSample.cs — new sample in the Special category with an embedded ClusteringProvider (decorator over IProvider) that clusters features per zoom level using a grid algorithm. Cache is per zoom level, correctly invalidated on data change, releases lock before awaiting builds (parallel zoom levels), and evicts faulted tasks so they are retried.
• docs/general/markdown/working-with-samples.md — new documentation page explaining how the sample system works and how to add a new sample.
• docs/general/mkdocs.yml — wires the new page into the Contributing nav section.