[RDSC-4241] Improving observability documentation

[ilianiliev-redis]

Ticket: https://redislabs.atlassian.net/browse/RDSC-4241

IMPORTANT: Do not merge or delete before the merge of https://github.com/RedisLabs/redis-data-integration/pull/2288 and the release of the respectful RDI version

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Note

Low Risk
Documentation-only updates that change how users are instructed to access RDI metrics endpoints; no product code or runtime behavior is modified.

Overview
Adds a new “Accessing the metrics” section that distinguishes VM vs Helm installs, documents the correct metrics endpoints for each component, and provides Helm values.yaml examples to enable Prometheus ServiceMonitors (with a note about discovery delays and a pre-1.16.0 collector caveat).

Removes the old per-section endpoint callouts for collector/stream processor, and updates the operator metrics section to note most metrics are standard controller-runtime metrics while highlighting the RDI-specific ones.

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[andy-stark-redis]

A few suggestions, mainly to fit better with docs heading structure and markup conventions. Otherwise, LGTM.

[andy-stark-redis]

LGTM