Clarify in docs that ClickHouse Cloud currently does not support cross region failover

[aashishkohli]

Thread for context: https://clickhouse-inc.slack.com/archives/C037AU5BP37/p1739464538986769

We have also seen this discussed with various customers such as RH recently: https://clickhouse-inc.slack.com/archives/C07MY85Q6UU/p1739383779621249

1. We should clarify in our docs that we currently do not support cross region failover, or clusters that span different regions (multi-region clusters).
2. We should state DR is possible using BYOB - exporting backups to a bucket in another region.
3. After the update ensure that AI search is correctly up-leveling this

[Blargian]

Potentially expand this page?

https://clickhouse.com/docs/en/cloud/reference/supported-regions

Update this page with a note relevant to Cloud:

https://clickhouse.com/docs/en/faq/operations/multi-region-replication