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]

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[ciarams87]

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

[sjberman]

The only downside of using mermaid versus manual is it doesn't render the prettiest or most balanced diagrams. (like the giant nginx-gateway namespace box). Unless you can create a manual image and translate into mermaid?

[ciarams87]

I'll see if I can tweak it a bit more, but yeah, it is an issue.

[ciarams87]

I've changed the alignment to Left Right from Top Down which has kind of improved it I think?

[sjberman]

Yeah, that does look a bit better. Only nitpick would be the AB components are below the C components.

[ciarams87]

I'll see if I can reorder them