Write FAQ about passkeys and how they should be used for existing & greenfield apps

[tcannonfodder]

Some questions to answer:

• What is/is not a passkey?
• How do you implement passkeys for various authentication frameworks (including legacy/roll your own)
• What do you do if your stakeholders/customers absolutely need passwords as a security blanket?

Reference materials:

• Passkeys support heartcombo/devise#5527
• https://www.passkeys.com/
• https://webauthn.guide/
• https://passkeys.dev

[tcannonfodder]

@htcarr3 👋 for this, let's add a docs/ folder in this Repo, with Markdown files inside it, and we'll link to it in the README