doc: put more "why"/"rationale" on architecture page

[mathetake]

Description:

We already have the dedicated "architecture" pages that explain each component control plane, data plain as well as the request flow that will be helpful for advanced users to understand "how it works" as well as for the contributors to understand the codebase.

However, the section generally misses "why" it is architected in a specific way. For example, why EG's extension servers are needed, why both router filter and upstream filter exist, etc

https://aigateway.envoyproxy.io/docs/concepts/architecture/system-architecture/