chore: update cluster docs

[danielamiao]

No description provided.

[Copilot]

Pull request overview

Updates the Momento Valkey cluster HTTP API documentation to reflect the newer /ec-cluster control-plane endpoints and adds coverage for additional cluster mutation operations.

Changes:

• Updates Create/Delete/List endpoint paths and request/response shapes to use /ec-cluster and a name field in the create payload.
• Adds new endpoint documentation for Describe Cluster and for update operations (replication group, shard config, replica count changes).
• Expands the Examples section with curl commands for the new/updated endpoints.

Comments suppressed due to low confidence (1)

docs/valkey/http-api.md:566

• Under # Examples, several headings duplicate earlier endpoint headings (e.g., ## Describe Cluster, ## List Clusters). This typically leads to duplicate entries in the rendered table-of-contents and auto-generated anchor IDs with suffixes (e.g., describe-cluster vs describe-cluster-1), which makes deep-linking harder. Consider renaming the example headings (e.g., "Describe Cluster (Example)") or otherwise differentiating them from the API reference headings.

## Describe Cluster

Get details for a specific cluster:

```bash
curl -H "Authorization: <token>" \
  "https://api.cache.cell-1-us-east-1-1.prod.a.momentohq.com/ec-cluster/my-cluster"

List Clusters

</details>



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[anp13]

This is not what we will see in response for Creating. This is a sample response for Creating now:

{
  "name": "test-ec-clusters",
  "enforce_shard_multi_az": true,
  "shard_count": 0,
  "replication_factor": 0,
  "shard_placements": [],
  "node_instance_type": "cache.m6g.xlarge",
  "status": "Creating"
}

I think we should not return any response for Create because it is weird that the fields are wiped out in the response. But today we return this.

[anp13]

nit: May be we should update this example to have 3 shards since the update shard config example assumes that we are starting with 3 shards and going to 5.