NGF: Update architecture overview

[ciarams87]

Proposed changes

Problem: The Gateway architecture document does not reflect the new control plane/ data plane split

Solution: Update the architecture document to reflect the new architecture. This PR also converts the existing diagrams into mermaid from PNG files.

Closes nginx/nginx-gateway-fabric#3234

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[github-actions]

✅ All required contributors have signed the F5 CLA for this PR. Thank you!
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[ciarams87]

I have hereby read the F5 CLA and agree to its terms

[bjee19]

looks like there might be some rebase/merging issues with other files included in the pr, but the gateway-architecture looks good to me! 🚀