register apps as handlers for https://i.delta.chat then document it

[Simon-Laux]

When apps register as handler for https://i.delta.chat then document it in https://github.com/deltachat/interface/blob/master/uri-schemes.md

[Simon-Laux]

We should do this (register for deep-linking in i.delta.chat on all platforms), especially since we are now switching the apps to output invite strings, it makes sense that when you have the app installed it does not open the webpage clicking on a link, instead going to the app immediately.

• core already can handle the invite links as of feat: qr::check_qr(): Accept i.delta.chat invite links (#5217) deltachat-core-rust#5219.
• android registers for it since register to handle i.delta.chat links deltachat-android#2922 (has it's own implementation convert it back skipping core, because core did not have it back when it was implemented)
• desktop issue (Register to handle i.delta.chat links deltachat-desktop#3657), though not sure yet if possible on desktop
• iOS issue Register to handle i.delta.chat links deltachat-ios#2065

Also we can think about changing the qr code displayed in DC to an invite link as well in core, depending on how likely it is that people try to scan it without deltachat installed with their camera app.

[r10s]

closed automatically by deltachat/interface#61 as initially, this issue was only about documentation, however, as there are dedicated issues for the platforms, it seems fine

otherwise, please do direct PRs