Docs: sync upstream clickhouse-operator docs updates

[mintlify]

Summary

Syncs the docs/ content from clickhouse/clickhouse-operator into products/kubernetes-operator/ following the merge of upstream PR #226 (commit 6346a75).

cc @GrigoryPervakov for review — original author of the upstream docs changes.

Changes

• guides/configuration.mdx — frontmatter title changed to Configuration, H1 re-added in body, added new External Secret section (~290 lines), and updated default resources example values (256Mi/1Gi → 512Mi/512Mi).
• guides/introduction.mdx — frontmatter title changed to Introduction, H1 re-added, internal links to the Replicated database engine page updated to /core/reference/engines/database-engines/replicated.
• guides/monitoring.mdx — new file covering controller-manager Prometheus metrics, TLS, ServiceMonitor, alerting rules and Grafana.
• guides/scaling.mdx — new file covering shard, replica, and Keeper scaling.
• install/olm.mdx — adds a trailing newline only.
• overview.mdx — drops the "Monitoring" bullet from the Guides section (kept matching upstream — note: upstream removed it but the new monitoring guide is reachable via the side navigation).
• reference/api-reference.mdx — adds AdditionalPort type and corresponding additionalPorts field on ClickHouseClusterSpec, and updates the resources field description to reflect the new SMP deep-merge semantics.
• navigation.json — adds the two new guides (monitoring, scaling) to the Guides group.

Context

Upstream PR #226 itself only modified internal/controller/keeper/templates.go, but the upstream docs/ tree had accumulated unmerged drift relative to this repo. Per the sync rules, all updated files under docs/ have been copied verbatim — any content issues should be fixed upstream first.

Known issues / conflicts to flag

• guides/monitoring.mdx links to /operations/server-configuration-parameters/settings#prometheus, which does not exist in this docs site. The equivalent page lives under a different path here. Fix upstream before this is resolved cleanly.
• overview.mdx dropped the "Monitoring" bullet from the Guides list while a new guides/monitoring.mdx page was added. The page is still reachable via the navigation, but the overview no longer references it — likely an upstream oversight worth a follow-up upstream PR.
• The mint validate step timed out in CI sandbox; broken-links was not run. Reviewers should rely on Mintlify's preview build for link validation.

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Note

Low Risk
Docs-only sync with a known broken cross-link in the new monitoring guide (/operations/...#prometheus); no runtime or security impact.

Overview
Documentation sync from upstream clickhouse-operator into products/kubernetes-operator/, with no operator code changes.

The configuration guide gets frontmatter title tweaks (H1 moved into the body), updated default container memory in the example (512Mi request/limit), new sections on spec.externalSecret, additionalPorts, version probe / upgradeChannel, and spec.settings.logger, plus internal links retargeted to /core/... paths.

New guides: monitoring.mdx (controller-manager Prometheus metrics, Helm ServiceMonitor, TLS/RBAC, PromQL and alerts) and scaling.mdx (replica/shard scaling, schema sync conditions, Keeper quorum scaling).

Navigation adds both guides under Guides; overview drops the Monitoring bullet (page remains in the sidebar). API reference documents AdditionalPort / additionalPorts and revises containerTemplate.resources to “applied as a whole” instead of SMP deep-merge. Introduction Replicated-engine links use /core/reference/....

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