OIDC parameters for Authentik

[AlphaJack]

I had to modify the OIDC discovery endpoint URL given by my IDP, because Miniflux automatically appends .well-known/openid-configuration, similar to goharbor/harbor#12535.

Maybe it is worth mentioning in the documentation that the ".well-known" part is automatically appended.

These parameters work for Authentik:

OAUTH2_PROVIDER=oidc
OAUTH2_CLIENT_ID=XXXXXX
OAUTH2_CLIENT_SECRET=YYYYYY
OAUTH2_REDIRECT_URL=https://miniflux.example.org/oauth2/oidc/callback
# discovery endpoint is        https://authentik.example.org/application/o/miniflux/.well-known/openid-configuration
OAUTH2_OIDC_DISCOVERY_ENDPOINT=https://authentik.example.org/application/o/miniflux/
# if not set, miniflux returns "Access forbidden" if the user doesn't exist
OAUTH2_USER_CREATION=1

[fguillot]

I have updated the documentation: https://miniflux.app/docs/howto.html#oauth2