Docs: Added a guide to submitting an iOS app with subscriptions

[MattHeaney23]

Motivation / Description / Changes introduced

• A common area of confusion is how to submit a new iOS app with subscriptions
• This leads to a lot of people submitting their subscriptions to Apple in the wrong way, which can result in their new app being rejected
• This guide leads people from uploading their app via Xcode, to checking everything is set up correctly with their subscriptions, to attaching those subscriptions to their new app release

Additional comments

• I am also working on a video to be attached to this document, but this will be added in a future PR
• This guide focuses on subscriptions, and does not cover other areas of setting up an app in App Store Connect, but there is scope to add this in the future
• This guide assumes subscriptions are already set up and tested in the app

Tests

• Typo and Prettier pass in the pre-commit hook