Add OIDC provider docs for IBM ISAM

[ghost]

This adds the docs for the IBM ISAM OIDC provider

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[austingebauer]

Thanks for this, @godvoigt! Are you able to sign the CLA?

[austingebauer]

This is looking good, but I think it's missing a few things. Could you add changes similar to what was done in https://github.com/hashicorp/vault/pull/16274/files?

[ghost]

Hi @austingebauer, I will take a look at your suggestion.

As for the CLA, it's the same as for hashicorp/vault-plugin-auth-jwt#222. I signed it, but the state is not updated here. Clicking on "Refresh it" only takes me to my Github profile.

[ghost]

@austingebauer I've added the files you requested similar to the SecureAuth implementation. I also fixed the CLA now (my company mail changed a while ago and I hadn't configured / verified the new one in GitHub)

[austingebauer]

Thanks, @godvoigt!