docs: add JWT validation guide

[AtharvaKathe18]

Description

This PR adds a JWT validation guide to the security documentation.

Motivation:
While JWT support exists in the API reference, there is no practical guide showing how to configure and use it in a gateway setup.

What changed:

• Added a new JWT validation guide under the security documentation
• Included examples for configuring issuer and JWKS (remote URL)
• Covered audience validation and claim propagation
• Added curl-based testing and troubleshooting guidance

Closes #730

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Change Type

/kind documentation

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Changelog

Add JWT validation guide to security documentation

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Additional Notes

This guide focuses on JWT validation (authentication) and intentionally avoids authorization logic to keep scope aligned with existing documentation patterns.

[AtharvaKathe18]

Hi, just following up on this PR.

I’d appreciate any feedback when you have time. Happy to make updates if needed.

[kristin-kronstain-brown]

@AtharvaKathe18 We recently had some JWT additions in other PRs. Can you review what's in there and see if there is anything your PR has that isn't there yet?
https://kgateway.dev/docs/envoy/latest/security/jwt/