Update docs to emphasize on puter login popup for websites

[reynaldichernando]

We got report that the puter login popup was unclear, and they didn't see it mentioned in the docs
We need to update the docs to mention this somewhere, maybe could just be a sentence along the lines of

• you need to get authenticated with your puter.com account when using puterjs
• if not, there will be this login prompt automatically handle for you
• or you can auth manually using puter.auth.signIn
• or if you use apps, you don't need to handle this

[reynaldichernando]

while writing this i'm also thinking some things, which we may consider explain

• apps -> auto auth
• websites -> manual auth
• workers -> no auth (workers call get charged to devs?)
• nodejs -> auth with token

[reynaldichernando]

we could also link to our security and permissions page, since that's where the info will live

[reynaldichernando]

we kinda have this already in https://docs.puter.com/security/
closing this one, unless we get feedbacks in the future