docs: add OIDC/OAuth2 concepts overview guide

[AtharvaKathe18]

Description

This PR adds an OIDC/OAuth2 concepts overview guide to the security documentation.

Motivation:
While kgateway provides support for OAuth2/OIDC and includes provider-specific guides (such as Keycloak) and a JWT validation guide, there is no central explanation of how authentication flows work in kgateway or how these approaches relate to each other.

What changed:

• Added a new OIDC/OAuth2 overview guide explaining authentication flows in kgateway
• Covered the Authorization Code flow and how it is handled at the gateway
• Included a comparison of JWT validation vs OIDC
• Provided guidance on when to use each approach
• Linked to existing JWT and Keycloak guides
• Added navigation entry under the security section

Closes #735

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Change Type

/kind documentation

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Changelog

Add OIDC/OAuth2 concepts overview guide to security documentation

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Additional Notes

This guide focuses on conceptual understanding and complements existing provider-specific guides without duplicating them.