docs: add generic oidc identity provider guide

[desimone]

Is your feature request related to a problem? Please describe.

Though we currently support generic OpenID Connect oidc providers, the functionality is not documented. Ideally, using a self-hosted provider like keycloak.

Describe the solution you'd like

Add an identity providers guide.

Additional context

• https://github.com/pomerium/pomerium/blob/658e80f2af09010d46ac7d4c28220c5821ea5930/internal/identity/providers.go#L28

[lukasmrtvy]

Here is really nice guide for integrating dex and pomerium: https://banzaicloud.com/blog/pomerium-authentication/ ( @desimone ) How is https://www.pomerium.io/reference/#identity-provider-service-account working for generic OpenID ? Thanks