This will be a nice addition to the currently supported features in Android (MOBILE-4410)

We should just copy the Firebase Dynamic Links approach (being deprecated) but that works, see attached links and relevant screenshots

https://firebase.google.com/docs/dynamic-links/operating-system-integrations#ios_integration_flowchart

So the approach for iOS would be:
1. The user clicks the "Get the mobile app" link
2. The user is redirected to our download page https://download.moodle.org/mobile?siteurl=https://mymoodlesite.com
3. The download page presents the link to Apple App Store
4. The user clicks the Apple App Store button, using JS we copy to the iOS clipboard (only iOS, detecting user agent) the site URL pre-pending something to the copied text indicating that it is a valid site URL: for example moodlesiteurl:https://mymoodlesite.com
5. The user opens the app, and the app analyses the clipboard to check for a copied text starting by moodlesiteurl:, if success, the user is automatically redirected to their Moodle site so they can enter their credentials.