[Docs] Improve site structure and diagrams

[graehren]

Requirements

• Work with technical writer to improve the site structure and organization
• Make the docs more searchable with clear and search optimized headlines
• Improve the diagrams to improve the readability

[graehren]

Look at issue #253 for example on improving docs for architecture with multiple service networks. Use https://github.com/aws/aws-application-networking-k8s/blob/main/docs/multi-sn.md as a starting point.

[zijun726911]

Done by #630