Developer Doc: Explain how NKG processes events

[pleshakov]

NKG configures NGINX as a response to changes in the cluster. Such changes come to NKG in a form of events about resources (ex. HTTPRoute is changed, an EndpointSlice is updated, etc.).

NKG processes those events and regenerates NGINX config if necessary.

Acceptance criteria:

• Explain how NKG processes events in the architecture doc https://github.com/nginxinc/nginx-kubernetes-gateway/blob/main/docs/architecture.md
• Explain the event batching mechanism.
• Explain when NKG will regenerate the config and when it will skip.
• Explain when NKG will reload NGINX and when it will not.
• Explain how quickly a change to resource will be reflected in NGINX, and what affects that time.
• Ensure that the documentation is not about the code, but rather than an algorithm, to make users understand them - they are the main target of that doc.

Note: such knowledge will help the user understand the behavior of NKG and use that knowledge to successfully manage NKG.

[mpstefan]

We talked today about changing this to the perspective of a developer looking to contribute to the project.