docs: Split Keeper page into sub-sections

[Harsh-pa-thak]

Fixes #5904

The Keeper page was a single ~1,400 line page mixing many unrelated concerns. This splits it into four focused sub-pages — Overview, Setup, Reference, and Guides — following the same pattern as the ClickHouse Operator section.

No content was changed, only re-organised.

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Note

Low Risk
Documentation-only reorganization; main risk is broken inbound links to the old /guides/sre/keeper/clickhouse-keeper or index.md URLs unless redirects are added elsewhere.

Overview
Replaces the single ~1,400-line ClickHouse Keeper page with four focused docs (Overview, Setup, Reference, Guides), aligned with the ClickHouse Operator section layout.

The former index.md content is removed and redistributed: high-level intro on Overview; configuration, run instructions, ZooKeeper migration, and quorum recovery on Setup; 4lw commands, HTTP control, feature flags, disks, Prometheus, and unsupported features on Reference; cluster walkthroughs, {uuid} paths, dynamic reconfiguration, and single-node expansion on Guides.

The Keeper category drops the Docusaurus generated index (link block in _category_.yml). Cross-links inside Setup now point at the new slugs (e.g. reconfiguration → Guides, HTTP control → Reference) instead of same-page anchors.

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