docs: Envoy AI Gateway v0.1 design doc

[yuzisun]

This PR documents the envoy ai gateway 0.1 release API and architecture design.

[mathetake]

meta question: should we call this simply v1 design doc or something like that? I guess it's not a proposal per-se but more of a like a design doc

[yuzisun]

meta question: should we call this simply v1 design doc or something like that? I guess it's not a proposal per-se but more of a like a design doc

true, rewording to v1 design doc

[mathetake]

Overall looking correct implementation pov. If we don't publish this on the website then I think we don't need to spend too much time on this (like wording backend vs provider etc) and i can approve after @missBerg takes a look.

we are going to have a similar page in the website like the high level Architecture right? @missBerg

[mathetake]

i think we also need an arrow from Envoy Extension Policy to Envoy Gateway Controller, just like the one from HTTPRoute?

[mathetake]

looking cool (and correct on how the rate limit works)!