Document Elasticsearch/OpenSearch sink support and setup

[kwannoel]

Context

Slack thread: https://slack.com/archives/C0AE8A81LE6/p1771087120973229

A user pinged directly in Slack, which suggests they could not find self-serve documentation for this connector workflow.

Problem

RisingWave appears to support Elasticsearch/OpenSearch sink integration in code, but there is no clear, user-facing documentation page that explains how to use it end to end.

Without a dedicated guide, users have to ask in Slack for basic usage and compatibility details, which increases support load and slows adoption.

Suggested Fix

Create a dedicated docs page for Elasticsearch/OpenSearch sink usage, and link it from the connector/navigation entry points.

Include:

• Supported engines/versions (Elasticsearch vs OpenSearch) and any compatibility notes.
• Required connector options and optional tuning parameters.
• A complete SQL example for creating a sink.
• Data type mapping and null/JSON behavior notes where relevant.
• Error handling/troubleshooting tips (common auth, endpoint, and schema mismatch issues).

Also add cross-links from related sink/connector docs so the page is discoverable via navigation and search.

Existing Documentation

• iceberg/data-types.mdx
• operate/manage-secrets.mdx
• sql/functions/aggregate.mdx
• sql/udfs/use-udfs-in-rust.mdx
• sql/udfs/use-udfs-in-java.mdx

Codebase References

Related PRs in risingwave:

• fix: correctly handle vis map in hash dispatcher after noop elimination (#24580)
• feat(expr): allow jsonb null values to be cast to scalar types as null
• feat: support sink payload as bytes into downstream mq
• fix: inaccurate join barrier processing time and description
• feat(iceberg): support alter iceberg table connector with

Source files:

• risingwavelabs/risingwave/src/connector/src/sink/elasticsearch_opensearch/elasticsearch_opensearch_client.rs
• risingwavelabs/risingwave/java/connector-node/risingwave-sink-es-7/src/main/java/com/risingwave/connector/ElasticRestHighLevelClientAdapter.java
• risingwavelabs/risingwave/java/connector-node/risingwave-sink-es-7/src/main/java/com/risingwave/connector/OpensearchRestHighLevelClientAdapter.java
• risingwavelabs/risingwave/proto/task_service.proto
• risingwavelabs/risingwave/src/risedevtool/src/task/task_db_ready_check.rs