Plugin Docs CLI: Extract TOC headings at build time

[sunker]

What this PR does / why we need it:

The Next.js website renders a "Table of contents" for every plugin docs page, listing the h2 and h3 headings in the page. Today those headings are extracted on every page render by re-parsing the full markdown body - a cost paid on the hot path for every page view on grafana.com.

This PR moves heading extraction to build time. The build command in @grafana/plugin-docs-cli now extracts h2/h3 headings from each page and writes them into the generated manifest.json under a new optional headings field on Page. Consumers can then build the TOC straight from the manifest with zero markdown parsing per request.

Extraction reuses parseMarkdown() from @grafana/plugin-docs-parser, so manifest heading IDs match the IDs the same pipeline produces at render time - anchors line up by construction.

Which issue(s) this PR fixes:

Fixes #

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.2644.26276784934.0
npm install @grafana/create-plugin@7.6.1-canary.2644.26276784934.0
npm install @grafana/plugin-docs-cli@0.0.11-canary.2644.26276784934.0
npm install @grafana/plugin-docs-parser@0.0.5-canary.2644.26276784934.0
npm install @grafana/plugin-meta-extractor@0.12.3-canary.2644.26276784934.0
# or 
yarn add website@5.6.1-canary.2644.26276784934.0
yarn add @grafana/create-plugin@7.6.1-canary.2644.26276784934.0
yarn add @grafana/plugin-docs-cli@0.0.11-canary.2644.26276784934.0
yarn add @grafana/plugin-docs-parser@0.0.5-canary.2644.26276784934.0
yarn add @grafana/plugin-meta-extractor@0.12.3-canary.2644.26276784934.0

[github-actions]

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[Copilot]

Pull request overview

This PR moves table-of-contents (TOC) heading extraction (h2/h3) from render time to build time by embedding extracted headings into the generated docs manifest.json. It leverages the existing @grafana/plugin-docs-parser pipeline (parseMarkdown) so heading IDs in the manifest match those produced during rendering, ensuring anchors remain consistent.

Changes:

• Add an optional headings field to the Page type in @grafana/plugin-docs-parser.
• Extract h2/h3 headings during docs scanning in @grafana/plugin-docs-cli and attach them to each file-backed page.
• Update scanner tests to validate heading extraction and omission behavior when no h2/h3 exist.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/plugin-docs-parser/src/types.ts	Extends Page with optional headings metadata for TOC generation.
packages/plugin-docs-cli/src/scanner.ts	Uses parseMarkdown() during scanning to extract and persist h2/h3 headings into the manifest page tree.
packages/plugin-docs-cli/src/scanner.test.ts	Adds coverage asserting headings are extracted (and omitted when absent).