Update authentication and personalization docs

[ethanpalm]

This PR updates the pages in the "Authentication and personalization" group.

Summary of changes:

• Combine conceptual information into a new Overview page that helps people choose between auth, partial auth, and personalization; introduces handshake methods and makes the info less hidden in tabs
• Combine individual set up articles for each handshake method into one article for auth, partial auth, and personalization. Makes info easier to find in sidebar without expanding folders
• Add some more info and examples to Sending data

closes https://linear.app/mintlify/issue/DOC-25/authentication-and-personalization-refresh

❗ This PR is currently blocked because we can't direct users to the authentication setting via hyperlink. The URL currently 404s. Do not merge until that is fixed. Resolved ✨

[mayankshouche]

left a few nits but overall looks great!

This PR is currently blocked because we can't direct users to the authentication setting via hyperlink. The URL currently 404s. Do not merge until that is fixed.

^^ this should also be resolved!