docs(toc): document noIndex behavior

[kadymov]

Problem

The documentation does not explain how indexing restrictions from toc.yaml, page metadata, .yfm, and hidden interact.

Changes

• document noIndex at the TOC root, on an individual page, and on a section;
• explain inheritance through the subtree and included TOCs;
• state that false cannot cancel an existing restriction;
• distinguish page frontmatter from the global .yfm docs-viewer.no-index setting;
• clarify that hidden controls navigation independently and does not set meta.noIndex;
• add matching examples and descriptions in Russian and English.

Verification

• built the complete RU/EN documentation with CLI 5.57.3;
• checked the generated TOC, metadata, and LLMS documentation pages;
• git diff --check.

Implementation: diplodoc-platform/cli#2245
Schema: diplodoc-platform/ajv#64

Task: DOCSTOOLS-4902

[github-actions]

✅ Documentation build

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Build logs

Warnings (2)

⚠️ en/guides/openapi.md: 168: YFM001 / inline-code-length Inline code length [Expected: 100; Actual: 265] [Context: " — the specification is displayed directly on the page (default value). If the size of the specification's json schema exceeds the value of ##maxOpenapiIncludeSize## (100 KB by default), the mode automatically switches to "]

⚠️ en/tools/docs/build.md: 52: YFM001 / inline-code-length Inline code length [Expected: 100; Actual: 109] [Context: " — a snapshot of the dependency graph (pages, included files, assets, missing paths, edge count), as well as "]