docs(readme): add granian as an alternative

[Zerotask]

Summary

• Update mangum website; current website just shows a blank page
• Add Granian as an alternative

Checklist

• [x] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [ ] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [x] I've updated the documentation accordingly.

[Kludex]

@Kludex looks a bit polluted, left an alternative suggestion

Grazie 🙏