Add SECRET_KEY environment or configuration variable as prerequisite for OIDC Authentication

[headofchoker]

I was trying to enable OIDC Authentication via Forgejo.

docker logs grampsweb output:

authlib.integrations.base_client.errors.OAuthError: unauthorized_client: client is not authorized
OIDC callback error for provider 'custom'

I've had 502 Bad Request on a page until GRAMPSWEB_SECRET_KEY and GRAMPSWEB_BASE_URL are written in docker-compose.yml

I hope adding such section as a prerequisite would make it more obvious for other users to enable OIDC.

[DavidMStraub]

Gramps Web will refuse to start, with a pretty explicit error message, if SECRET_KEY is not set. This is not only required for OIDC, but for any authentication.

https://github.com/gramps-project/gramps-web-api/blob/44376d5a0ddef5c412425156f9beb6871fdf9240/gramps_webapi/app.py#L108-L112

[DavidMStraub]

Also, SECRET_KEY and BASE_URL are used in all OIDC examples in the docs...