Port "testing push notifications" documentation from zulip-mobile repo.

[laurynmm]

CZO discussion: #documentation > contributor testing push notifications doc

Currently, the main documentation for testing push notifications for mobile developers is at https://github.com/zulip/zulip-mobile/blob/main/docs/howto/push-notifications.md.

While it is linked to in the how to documentation for Push notifications iOS simulator, it would be good to port that documentation to this repository and update zproject/dev_settings.py in the zulip/zulip repository following that change.

[rovertrack]

Can I do this ???