RDSC-5610: Multi-source documentation

[stoyanr]

See https://redislabs.atlassian.net/browse/RDSC-5610

Enhances #3841 with numerous changes and additions. I couldn't update the original PR since I don't have write permissions to this repository.

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Note

Low Risk
Documentation-only changes; no application code. Incorrect secret or API migration steps could confuse operators, but the content describes existing RDI behavior rather than changing runtime systems.

Overview
Documents multi-source RDI pipelines: one pipeline can ingest from several databases into a single Redis target, with a collector, credentials, and streams per source. Adds the dedicated guide Multiple sources in one pipeline (naming rules, config.yaml examples, jobs/server_name, --source for start/stop/reset, monitoring, and legacy resource names after upgrade).

Secrets and deploy are rewritten around database-scoped keys (USERNAME, PASSWORD, …) and redis-di set-secret … --db <source|target>, with env vars like ${MYSQL_DB_PASSWORD} and K8s secrets named <source>-db / <source>-db-ssl. Legacy SOURCE_DB_* / TARGET_DB_* remain documented for single-source pipelines that predate the change.

Product positioning updates remove the “single source only” decision-tree branch and state that multiple sources are supported; architecture, pipelines, DLQ keys (<source>.<schema>.<table>), observability (per-collector metrics URLs), HA examples (collector-mysql), VM standby behavior, and API v1 vs v2 (multi-source requires v2) are aligned. Flink job advanced matchers (lists/regex: on server_name, db, schema, table) are noted. Prepare-database pages and pipeline-config examples are updated to match the new secret model.

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