docs: refresh observability concepts foundation

[killme2008]

What changed

Refresh the Nightly concepts foundation around the questions teams ask when evaluating an observability backend.

• Rework why-greptimedb.md around operating problems and their corresponding mechanisms instead of presenting an architecture inventory.
• Explain one columnar engine with separate tables, schemas, indexes, TTLs, and compaction settings for different signal types.
• Show how real-time monitoring and historical analysis share one system while retaining separate workload controls.
• State ingestion and query boundaries without implying full compatibility with source systems.
• Separate open-source capabilities from GreptimeDB Enterprise features.
• Clarify the time index, Tag/Timestamp/Field roles, merge modes, table engines, storage providers, and recovery responsibilities.
• Redraw the time-series table model and add localized diagrams for the unified signal model and shared monitoring workloads.
• Preserve published routes and fragments. Add repository guidance that AnchorAlias protects published fragments, not intermediate headings from an unmerged PR.

Stack

• Base: main
• Dependency: none; docs: clarify Observability 2.0 data model #2739 and docs: add Observability 2.0 model diagrams #2742 are already merged into main
• Position: Stack PR 1
• Dependent PR: docs: correct observability concepts and terminology #2741

Scope

• Documentation versions: Nightly only; released versions are unchanged
• Languages: English and Chinese
• Repository guidance and CI: narrow updates for AnchorAlias handling and the front-matter exception for AGENTS.md

Verification

• git diff --check origin/main...HEAD
• pnpm dlx markdownlint-cli <changed files> --config .markdownlint.yaml
• pnpm exec typos <changed files>
• DOC_LANG=en pnpm check:links
• DOC_LANG=zh pnpm check:links
• xmllint --noout for the new SVG files
• Rendered and inspected the English and Chinese diagrams for clipping, overlap, and text alignment
• Reviewed the complete diff for versioned docs, generated files, and lockfile changes

Both strict link checks completed their production builds successfully.

Checklist

• I verified the content against the applicable GreptimeDB version.
• I updated the relevant documentation versions and languages, or explained why not.
• I checked changed links and anchors.
• Navigation is unchanged; existing routes remain in place.

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