Format admonition blocks and fix relative path references

[radium-v]

Pull Request

This PR merges into users/janechu/replace-docusaurus-with-11ty, to be included with #7260.

📖 Description

Adds a custom markdown-it plugin for Docusaurus-style admonition blocks (:::tip, :::note, :::warning, :::important) so that existing content using this syntax renders correctly in the Eleventy-based site. Also registers the build/ directory as a workspace package so that shared build utilities (e.g., get-package-json) can be referenced by name instead of fragile relative paths.

• Add sites/website/plugins/admonitions.js — a markdown-it core rule that converts :::type … ::: fenced blocks into styled alert <div>s.
• Wire the plugin into eleventy.config.js via amendLibrary.
• Add build/package.json (@microsoft/fast-build) and include build in root workspaces.
• Update generate-docs.cjs to import from @microsoft/fast-build/get-package-json.js.
• Fix alert CSS: add missing semicolon, bottom margin, and collapse margins on last child.

📑 Test Plan

• Verified the Eleventy build completes without errors (npx @11ty/eleventy).
• Visually confirmed admonition blocks render with the correct styling and headings.

✅ Checklist

General

• I have tested my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.