Update references to Gateway API spec in NGF docs

[shaun-nx]

Proposed changes

The basic routing docs for has multiple references to Gateway and HTTPRoute specs in the GatewayAPI docs.
These references currently don't take the user to the exact field that is references.
The goal of this change is to update those references

Closes nginx/nginx-gateway-fabric#3975

Checklist

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• I read the Contributing guidelines
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• My content changes adhere to the F5 NGINX Documentation style guide
• If my changes involve potentially sensitive information¹, I have assessed the possible impact
• I have waited to ensure my changes pass tests, and addressed any discovered issues

Footnotes

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