Cleanup and reorganize the contribution documentation

[orpiske]

Once we merge apache/camel#10476, which will change the way we build, we should adjust the contribution documentation to reflect that.

My suggestion is to do the following:

• Remove the Camel Core specific contribution details from the main website and add it to the Core contributing guide
• Describe that each sub-project may have slightly different contribution guidelines (due to the specific nature of each of them)
• Create a contributing guide if missing and/or add a link to it from the main website.

[orpiske]

Also add a note about tickets labeled easy on the ASF Jira.