Improve docs about OIDC

Hello guys,

Just a small improvement for the docs:
If the OIDC Provider is also managed by Pangolin and thus the URL is pointing to the Host, you might want to add the URL of the OIDC Provider as an alias of gerbil.

### Example:

```YAML
gerbil:
image: fosrl/gerbil:1.1.0
container_name: gerbil
restart: unless-stopped
depends_on:
pangolin:
condition: service_healthy
command:
- --reachableAt=http://gerbil:3003
- --generateAndSaveKeyTo=/var/config/key
- --remoteConfig=http://pangolin:3001/api/v1/gerbil/get-config
- --reportBandwidthTo=http://pangolin:3001/api/v1/gerbil/receive-bandwidth
volumes:
- ./config/:/var/config
cap_add:
- NET_ADMIN
- SYS_MODULE
networks:
default:
aliases:
- idp.example.com
ports:
- 51820:51820/udp
- 443:443 # Port for traefik because of the network_mode
- 80:80 # Port for traefik because of the network_mode
```

Cheers