feat: Add a landing page for NGINX Gateway Fabric

[ADubhlaoich]

Proposed changes

This commit adds a landing page for NGINX Gateway Fabric. Custom landing pages allow us to draw a reader's attention towards the documentation they would find most useful as both a prospective or returning user, such as evergreen reference material.

Checklist

Before sharing this pull request, I completed the following checklist:

• I read the Contributing guidelines
• My branch adheres to the Git conventions
• 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

1. Potentially sensitive information includes personally identify information (PII), authentication credentials, and live URLs. Refer to the style guide for guidance about placeholder content. ↩

[github-actions]

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[ADubhlaoich]

Added some detail from the design page to the design section of the landing page, linking to the page itself. Took the pull request out of draft since this is now a MVP.

I don't want to treat the conversation around the design information for NGINX Gateway Fabric to become a blocker for this landing page when we can iterate on it relatively quickly.

[sjberman]

I think https://docs.nginx.com/nginx-gateway-fabric/traffic-security/integrate-cert-manager/ is going to be the more common use case than backend traffic. I believe users care more about securing traffic from the client to nginx (ingress traffic), versus traffic from nginx to the backend (which is already in the cluster).