Ntitoras/user 985 add expo passkeys guide

[AlexNti]

Important

🔎 Previews:

• https://clerk.com/docs/pr/1702/references/expo/passkeys

Explanation:

This PR:

Introduces the docs on how a user will be able to add Passkeys authentication/registration at their Expo application.

⚠️This feature is currently hidden behind a feature flag and only enabled for beta users that have requested access to it.

[github-actions]

Hey, here’s your docs preview: https://clerk.com/docs/pr/1702

[nikospapcom]

@alexisintech we did another pass with @Nikpolik and we did some minor changes
I'd appreciate a review here :)
let me know if you need something