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

[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! 🚀

[ADubhlaoich]

Generally LGTM, though I have some non-blocking suggestions:

I'd break the new block of text into separate paragraphs. It's a dense read at the moment, so line breaks would help a lot.

I like the new diagram (And the fact it's written with Mermaid): I think the Kubernetes API object could do with a lighter colour, however.

The black text on a dark purple background is difficult to read: with the size of the diagram rendered in the browser, we want to make sure that text is as legible as possible.

[sjberman]

@ADubhlaoich One thing I'll note with colors, is that in the past we found it difficult to make Mermaid look good in both light and dark modes in browsers. It's an art to figure out, pun intended.