Add Zitadel to documentation

[samumatic]

Is your feature request related to a problem? Please describe.
I struggle to get Zitadel working with the Jellyfin SSO Plugin, as described in #168 (comment).

Hey, i have some problems implementing zitadel with the plugin. I chose the Web(OIDC) -> Code type for it. After setting everything up, i set the redirect settings to: "https://jelly.lab.domain.de" - my jellyfin URL. But after clicking the button for "https://jelly.lab.domain.de/sso/OID/start/zitadel" the JSON answer states that:

The requested redirect_uri is missing in the client configuration. If you have any questions, you may contact the administrator of the application.

But in the URL that part seems ok: redirect_uri=https%3A%2F%2Fjelly.lab.domain.de%2Fsso%2FOID%2Fredirect%2Fzitadel

How did you set it up? Did i made a mistake here?

Describe the solution you'd like
Add the authentication provider Zitadel to the documentation, as considered in #31 (comment).

Describe alternatives you've considered
Just use another authentication provider - Authentik.

Additional context
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[samumatic]

Ok, I fixed it by specifying the exact path https://jelly.lab.domain.de/sso/OID/redirect/zitadel.
This seems to be a problem on the zitadel side rather than the plugin side, but a note in the documentation would be useful.

[9p4]

The docs all do say to use the full redirect URL. There's effectively no difference in the Zitadel setup from any other provider.