Phase 2: #47282 Improving Contributor On-Boarding Documentation

What does this MR do?

This MR resolved the changes requested in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19565. Tracked under a new MR due to the number of conflicts.

Are there points in the code the reviewer needs to double check?

• This needs to be reviewed by the relevant parties involved in #47282 (moved) before merging.
• All previous headings still exist in the contributing.md.
• No spelling mistakes are present.
• All hyperlinks are in a working condition.
• No content previously in the contributing.md is completely missing from the new locations.

Why was this MR needed?

To make the changes as discussed in both https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19565 and https://gitlab.com/gitlab-org/gitlab-ce/issues/47282 to help improve the onboarding process.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Conforms to the code review guidelines
  • Has been reviewed by a Backend maintainer
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them

What are the relevant issue numbers?