Update index.md

[tajkirkpatrick]

First draft rewrite.

Still missing a few instructions such as, node version disclaimer, polyfill disclaimer, setting up the astro.config.mjs in server mode (and accompanying adapter), rewording the callback URL explanation, and explaining the authOptions object that needs to be created by the end user as well as what actions to take when changing the prefix from the default "/api/auth" prefix.

NOTE:

• It's a good idea to open an issue first to discuss potential changes.
• Please make sure that you are NOT opening a PR to fix a potential security vulnerability. Instead, please follow the Security guidelines to disclose the issue to us confidentially.

☕️ Reasoning

🧢 Checklist

• Documentation
• Tests
• Ready to be merged

🎫 Affected issues

Please scout and link issues that might be solved by this PR.

Fixes: INSERT_ISSUE_LINK_HERE

📌 Resources

• Security guidelines
• Contributing guidelines
• Code of conduct
• Contributing to Open Source

[TheOtterlord]

Thanks! This looks awesome. Added a few minor notes for syntax highlighting & info card markup