docs(config): document supported VikingDB regions (add ap-southeast-1 from #2297)

[r266-tech]

Merged PR #2297 added ap-southeast-1 host mappings to the VikingDB clients, but the configuration guide's volcengine vectordb backend example only ever shows region: cn-beijing and never enumerates which regions are supported — so users outside Beijing (e.g. the new Southeast Asia region) have no way to discover the option from the docs.

This adds a short supported-regions note (EN + ZH) inside the existing volcengine vikingDB example block. The region set is sourced from openviking/storage/vectordb/collection/volcengine_clients.py:

• ClientForConsoleApi._global_host and ClientForDataApi._global_host both enumerate cn-beijing, cn-shanghai, cn-guangzhou, ap-southeast-1.

The note is qualified ("when no explicit host is set") since the clients do host if host else _global_host[region], so a custom-host deployment isn't restricted to the built-in region keys.

Pure docs, no code change.

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🎫 Ticket compliance analysis ✅ 2297 - Fully compliant Compliant requirements: (This PR is a docs follow-up to fix(vectordb): add ap-southeast-1 region host mappings for VikingDB #2297, which fulfilled the above requirements)

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