[Update] User Docs - Pay join page needs 1.5.0 UI updates

[Bas02]

Hey,

Britt made a request in #975 to extract Payjoin from user docs.
Pay join also needs updates to new 1.5.0 UI in Lightmode and rewriting for the new UI.
Maybe combine these two issues.

[bitcoinheiro]

Want to confirm before starting my research, would it be possible to add support for samourai's paynim to store owners? Since Sparrow adopted it as well, would probably be a way to incentivize "payjoin" from their users with merchants running the plugin. Thanks for your help.