Explain the differences between the email, Google, and GitHub logins for Community Cloud

[sfc-gh-dmatthews]

@nealmcb raised an issue (#726) about the clarity of purpose between the different account types on Streamlit Community Cloud: email, Google, and GitHub.

You can choose to log in via either a GitHub or Google account when creating a Community Cloud account. (explain why it might matter which one you use).

This has started to be documented with new features but needs to be more comprehensively introduced from the beginning/top of the Community Cloud documentation.