Document Camel Quarkus build items #2900

[AzazelSensei]

Adds a contributor-guide page that lists the core and Camel Main build items, in the same spirit as the Quarkus all-builditems catalog.

I also linked it from the custom components guide and the contributor nav.

Fixes #2900

[jamesnetherton]

Thanks @AzazelSensei. I probably should have added more info to the issue.

This task is somewhat more complex than just adding a static doc page.

• There are other extensions outside of the extensions-core/core that provide their own build items. So they also need to be listed
• The page needs to be built dynamically so that it stays fresh. Not sure what the best way to do that it is. But it needs to be unobtrusive and not impact project build. Maybe something added to tooling/maven-plugin where we have various mojos for manipulating docs.