Add Astro to project list

[BboyGT]

Summary

• Add Astro to the open source project list with its contribution page and relevant tags.

Validation

• corepack pnpm build

Addresses #1

[Sean-Kenneth-Doherty]

I validated this addition locally and it looks good to me.

Checks run:

• git diff --check main...HEAD passes
• pnpm build passes
• GitHub API reports withastro/astro is public, unarchived, enabled, and has open issues
• https://github.com/withastro/astro/contribute returns HTTP 200
• avatar URL returns HTTP 200 image/png
• gh search issues --repo withastro/astro --state open --label "good first issue" --limit 5 found current beginner-friendly issues, including #14013, #14657, and #14665
• built dist/index.html contains the Astro card/link once

This is a clean source-only project-list addition for #1.