Cleanup and reorganize the contribution documentation

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](https://github.com/apache/camel-website/blob/aa94dec2af6ece1e963fbb116598a8f3a9be1f71/content/community/contributing.md) 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.

Check the status of apache/camel#10476 first, then read content/community/contributing.md in the main website. Compare its Camel Core-specific details with the Core contributing guide and check which sub-projects lack guides or links. Done means the main guide reflects the new build approach, points to sub-project guidance, and no longer contains Core-specific details.

Written by the indexing model from the issue text.