Add InfluxDB 3 Cloud to the "Which InfluxDB 3 should I use?" decision guide

Nobody has claimed this yet.

## What

`/influxdb3/which-influxdb-3/` does not cover [InfluxDB 3 Cloud](https://docs.influxdata.com/influxdb3/cloud/). The decision guide should include it as an option so readers choosing a hosted, managed InfluxDB 3 service can find it.

## Why

InfluxDB 3 Cloud is documented at `/influxdb3/cloud/` as the fully managed, cloud-hosted version of InfluxDB 3 Enterprise. It is an early access release offered to select customers, and access is requested through the [product page](https://www.influxdata.com/products/influxdb3-cloud/).

Today the decision guide routes readers who want a managed, single-tenant cloud service to InfluxDB Cloud Dedicated only, so a reader who wants managed InfluxDB 3 Enterprise never learns that InfluxDB 3 Cloud exists.

## Proposed changes

- Add InfluxDB 3 Cloud to the comparison table with its architecture (hosted InfluxDB 3 Enterprise), tenancy (single-tenant), operations (managed by InfluxData), and query languages (SQL and InfluxQL).
- Cover it in the prose that recommends deployments. The page currently says to use InfluxDB 3 Enterprise for new production workloads; InfluxDB 3 Cloud is the managed way to run that same product.
- Pair every mention with the early access qualifier and the request-access link, so readers do not expect self-service signup. Write the qualifier consistently (ideally a shared shortcode) so it is easy to find and remove when the product reaches general availability.
- Keep InfluxDB Cloud Dedicated in the table. It is generally available and current customers need to find it by name.
- Describe the two accurately as separate products. InfluxDB 3 Cloud is a hosted InfluxDB 3 Enterprise cluster; Cloud Dedicated is a distributed InfluxDB 3 cluster with its own architecture and its own CLI (`influxctl` vs `influxdb3`). Do not present Cloud Dedicated as a deployment mode of Enterprise.

## Notes

Early access provides a focused subset of InfluxDB 3 Enterprise capabilities, so verify per feature against the InfluxDB 3 Cloud docs before claiming parity in any comparison row. For example, the Processing engine is documented for Core and Enterprise only.

## Related pages with the same gap

These also enumerate the InfluxDB 3 products without InfluxDB 3 Cloud. Worth a follow-up, separate from this issue:

- `/platform/` (product lineup and the version comparison table)
- `/platform/identify-version/` (Understanding InfluxDB products)

Start with the /influxdb3/which-influxdb-3/ decision guide and review the InfluxDB 3 Cloud documentation at /influxdb3/cloud/. Update the comparison and deployment guidance while checking each capability against the Cloud docs. Done means the guide distinguishes Cloud from Cloud Dedicated and consistently includes the early-access qualifier and request-access link.

Written by the indexing model from the issue text.