Redesigning website diagrams to be more cohesive

Many of the diagrams on the knative website are not visually cohesive. The diagrams should be updated to follow the same color scheme as the website and also use consistent font, box shape etc.

Child issues could be created for the following diagrams to start:
https://knative.dev/docs/community/
https://knative.dev/docs/concepts/
https://knative.dev/docs/concepts/eventing-resources/brokers/
https://knative.dev/docs/serving/architecture/
https://knative.dev/docs/serving/request-flow/
https://knative.dev/docs/eventing/event-mesh/

Additionally, alt text descriptions should be included for each diagram.