docs: update getting-started to reflect OAuth signup (no more waitlist)

[stackbilt-admin]

Signups are now open via GitHub and Google OAuth. The getting-started guide should be updated to reflect the actual onboarding flow:

1. Go to stackbilder.com and click "Start building"
2. Create account with GitHub or Google
3. Land on dashboard → create first flow or generate an image
4. Free tier: 3 scaffolds/mo, 5 images/mo — no credit card required

Remove any references to waitlist or "coming soon" messaging.