Push Notifications not working or Inadequate Documentation on setting up FCM GCM push with own Sygnal server

### Steps to reproduce

1. I updated pusher from https://matrix.org/_... to https://myserver/_....
2. I replaced google-services.json with version from my Firebase account created for app with my own apps package name
3. I setup Sygnal to connect to Dendrite

But I can NOT get any notifications on android phone.

### Outcome

#### What did you expect?
I expect to get some notification on android phone when app gets a new message.

#### What happened instead?
Nothing.

I don't see any error messages in sygnal.log or in dendrite log when I'm sending the message and no notification comes up.

### Your phone model

Samsung Note9

### Operating system version

Android 8.1

### Application version and app store

Element 1.4.16-dev olm 3.2.10 matrix sdk 1.4.16

### Homeserver

Dendrite github

### Will you send logs?

Yes