Please add the callback URL to OpenID connect docs

I just set up an OIDC IDP with [pocket-id](https://github.com/stonith404/pocket-id) that docspell can use to authenticate, and one thing that had me scratching my head at first was that there's no mention of the "final" URL on the docspell side that the authentication process will redirect to.

While it can be discovered with browser devtools or by reading the source code, I think it's better to just mention it: It's `https:///api/v1/open/auth/openid//resume` (:

Would you mind adding that to the documentation?