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.

Note that it now documents 2 PRs that are not yet merged, but should be before the release:

• https://github.com/RedisLabs/redis-data-integration/pull/2782
• https://github.com/RedisLabs/redis-data-integration/pull/2791

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Note

Low Risk
Documentation-only changes; operational risk is misconfiguration if published before the referenced RDI product changes ship.

Overview
Documents multi-source RDI pipelines: one pipeline can ingest from several source databases (each with its own collector, secrets, streams, and metrics), with a new Multiple sources in one pipeline guide covering naming, config.yaml, jobs (server_name), add/remove sources, per-source stop/start/reset, monitoring, and legacy naming after upgrade.

Secrets and deploy are rewritten around per-database keys (USERNAME, PASSWORD, TLS keys) and redis-di set-secret … --db <source|target>, with env vars like ${MYSQL_DB_PASSWORD}; K8s examples move from source-db to <source>-db. Legacy SOURCE_DB_* / TARGET_DB_* remain for single-source or pre-upgrade sources.

Product defaults and fit: architecture, FAQ, pipeline config, and upgrade pages state processors.type defaults to Flink (set classic explicitly to opt out), including a 2.0.0 upgrade warning that undeployed type pipelines move to Flink on next deploy. The “when to use” embed and decision tree drop the single-source-only gate and allow one or more sources.

Cross-cutting updates cover API v2 requirement for multi-source, DLQ/stream naming with source segments, Flink job matcher lists/regex:, observability per-collector URLs, HA/standby VM behavior, and refreshed database prep guides (MongoDB keystore, MySQL TLS, Snowflake/Spanner secrets, etc.).

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[andy-stark-redis]

Quite a few suggestions but they are easily applied, so I'll approve to avoid delays.

[zhivko-kirishev-redis]

Everything about the automatic population of the ssl properties as well as the default Flink processor looks good

[nicoweidner]

I left a few comments. We can tackle them in a followup so as not to block the release, none of them are critical

[nicoweidner]

Thanks for addressing all comments, looks good to me now!