Migrate site from Hugo to Astro and add CI/CD workflows for GitHub Pages

[sudheer]

Motivation

• Replace the existing Hugo site with an Astro-based site to modernize the frontend and developer tooling.
• Move content into an Astro-friendly structure and introduce typed content collections for safer content handling.
• Add GitHub Actions to provide automated type/content checks, builds, and per-PR preview deployments to GitHub Pages.
• Provide a bootstrap workflow and documentation to initialize and manage the gh-pages branch for previews and production.

Description

• Add a complete Astro project including package.json, package-lock.json, astro.config.mjs, tsconfig.json, and src/ containing layouts, components, pages, styles, and content files.
• Migrate legacy content from the old content/ tree into src/content/ as Markdown files and introduce src/content/config.ts with astro:content schemas for posts and micro collections.
• Add GitHub Actions workflows: astro-ci.yml to run npm run check and npm run build on PRs and pushes, astro-preview.yml to build and deploy PR previews to gh-pages/preview/pr-<number>/ and comment the preview URL on the PR, astro-pages.yml to build and deploy production to the gh-pages root, and bootstrap-pages-branch.yml to create a bootstrap gh-pages branch if missing.
• Remove the old hugo.yml workflow and add MIGRATION_NOTES.md documenting the Hugo → Astro migration, Pages settings, and the preview/production deployment behavior.

Testing

• CI is configured to run npm run check and npm run build on pull requests and pushes via .github/workflows/astro-ci.yml.
• The PR preview workflow astro-preview.yml is set to build the site and deploy previews under gh-pages/preview/pr-<number>/ and post the preview URL as a PR comment when reachable.
• The production workflow astro-pages.yml builds and deploys dist/ to the gh-pages branch while preserving preview folders using clean-exclude: preview.
• The workflows were added in this change; no automated workflow runs are included in this patchset (workflows will run once the branch is pushed and PR workflows are triggered).

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Codex Task