Push notification issues and documentation

### Steps to reproduce

1. Where are you starting? What can you see?
- I am running a matrix-synapse homeserver and have a custom build of the element-ios app.
- I am **not** running my own push notification server, such as sygnal, but have kept the default
`static let serverConfigSygnalAPIUrlString = "https://matrix.org/_matrix/push/v1/notify"` setting in the build pointing to the matrix run notification server.
- I cannot get push notifications to show up in the app. I have tested this in the simulator, local xcode build/installs, and via testflight.

3. Current debugging steps

- Pointing the element-ios app to my homeserver works fine for push notifications.
- Pointing any app (custom build or element-ios) I can see pushers appearing/disappearing in the homeserver DB `pushers` table as expected.
- I can reach the matrix notification sever from the homeserver instance. (For example, `curl https://example.org/_matrix/push/v1/notify` returns a 405, ie the server can be reached).
- I have configured certifications for push notifications as described in the apple developer docs, or outlined [here](curl https://example.org/_matrix/push/v1/notify).

5. More steps…
I have reached out in the element-ios:matrix.org room and have gone through all the available documentation for element-ios, sygnal and synapse.

I can't tell if this is a bug in element-ios or there if the documentation is lacking on the subject. Thank you!

### Outcome

#### What did you expect?

Expect to see notifications.
#### What happened instead?
No notifications.

### Your phone model

IphoneX

### Operating system version

15.3.1

### Application version

1.6.6

### Homeserver

Synapse 1.52.0

### Will you send logs?

Yes

No source file or test is named. Start by reviewing the reproduction details and the existing Element iOS, Synapse, and Sygnal documentation, then determine whether the failure is in the app or in the setup guidance. Done means either push notifications work for the described custom build or the missing configuration is documented clearly.

Written by the indexing model from the issue text.