RHOAIENG-36426: Docs for configuring BYOIDC providers

[mpandya15]

Description

This PR adds docs for configuring external identity providers with OpenShift AI by using the centralized Gateway API (this is a Tech Preview feature). Content additions cover:

• About the feature
• Configuring steps (with procedures on authenticating, verifying, and testing the setup)
• Troubleshooting

Jira: https://issues.redhat.com/browse/RHOAIENG-36426
Doc Draft: https://docs.google.com/document/d/1XiqC8ShfWjGS_EDhLDQcp9pxBHh9qDYRghmsxUiXXqU/edit?disco=AAABtowOAq4&tab=t.ahaku8qlzfjl

Preview: https://opendatahub-documentation--1049.org.readthedocs.build/en/1049/managing-odh/index.html#configuring-external-oidc-provider_managing-odh

How Has This Been Tested?

• QE/SME review and approval on the docs draft and this PR
• Peer review feedback

Merge criteria:

• The commits are squashed in a cohesive manner and have meaningful messages.
• The QE/SME has reviewed all doc updates and verified that the changes work

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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