Issue on docs

[caleighc]

Path: /onchainkit/fund/get-onramp-buy-url

Needs to be updated to reflect use of session tokens instead of projectId

[wearedood]

I'd like to help with this documentation update. Could you provide more details about:

1. Which specific documentation file needs to be updated?
2. What the current projectId implementation looks like vs. the new session tokens approach?
3. Are there any examples or references for the session tokens implementation?

I'm familiar with OnchainKit and would be happy to update the documentation to reflect the correct usage.