Move event source docs to developer guide

[abrennan89]

Clean up and move:

• https://dev-knative.netlify.app/development/eventing/sources/ping-source/
• https://dev-knative.netlify.app/development/eventing/sources/sinkbinding/
• https://dev-knative.netlify.app/development/eventing/sources/kafka-source/

Remove:

• https://dev-knative.netlify.app/development/eventing/sources/apache-camel-source/

[snneji]

/assign