fix(plugin-docs-cli): limit doc page nesting to 3

[sunker]

What this PR does / why we need it:

Adds the FS-7 max nesting depth rule to @grafana/plugin-docs-cli with a limit of 3 levels (so a/b/page.md is fine but a/b/c/page.md is too deep). Reports info during serve and error under validate --strict.

We need a hard cap because the SidebarNav component on the marketing website only looks sensible up to a handful of nesting levels - past that the nav gets cramped and pages become hard to discover. Better to stop authors before they ship a structure the rendered nav can't handle than to paper over it in the website.

Which issue(s) this PR fixes:

Part of the docs validation epic.

Special notes for your reviewer:

📦 Published PR as canary version: Canary Versions

✨ Test out this PR locally via:

npm install website@5.6.1-canary.2646.26441606068.0
npm install @grafana/create-plugin@7.6.1-canary.2646.26441606068.0
npm install @grafana/plugin-docs-cli@0.0.11-canary.2646.26441606068.0
npm install @grafana/plugin-meta-extractor@0.12.3-canary.2646.26441606068.0
# or 
yarn add website@5.6.1-canary.2646.26441606068.0
yarn add @grafana/create-plugin@7.6.1-canary.2646.26441606068.0
yarn add @grafana/plugin-docs-cli@0.0.11-canary.2646.26441606068.0
yarn add @grafana/plugin-meta-extractor@0.12.3-canary.2646.26441606068.0

[github-actions]

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[Copilot]

Pull request overview

Adds a new filesystem validation rule to @grafana/plugin-docs-cli to cap documentation page nesting depth at 3 path segments from the docs root, surfacing as info during serve (non-strict) and error during validate --strict.

Changes:

• Introduces Rule.MaxNestingDepth in the validation rule registry.
• Implements max-nesting-depth checking in the filesystem rule runner (checkFilesystem).
• Adds unit tests covering compliant depth, strict/non-strict severities, and per-file reporting.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
packages/plugin-docs-cli/src/validation/types.ts	Adds the new max-nesting-depth rule identifier to the central Rule list.
packages/plugin-docs-cli/src/validation/rules/filesystem.ts	Implements the nesting depth calculation and emits diagnostics based on strict.
packages/plugin-docs-cli/src/validation/rules/filesystem.test.ts	Adds test coverage for depth limits and severity behavior.