Improve contributor on-boarding experience

Currently, the contributor on-boarding experience is spread across multiple locations and doesn't include all useful information. This issue has been created to improve that experience so future & current contributors have information in one central location.

Tasks included in Phase 1 & 2 (Complete):

• Create a new contributing section in the docs with detailed information as seen fit. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20682
• Make contributor.md more concise https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20682
• Move the 'Code of Conduct' on about.gitlab.com/contributing to raise its importance edstub207/www-gitlab-com@deac13de
• Keep current headers in the CONTRIBUTING.md file since we link to those from various places. Their content should just be moved to the documentation and replaced with a link to it. https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20682

Tasks to be completed (Phase 3):

• Improve links between contributing.md, readme.md, about.gitlab.com/contributing & new docs.
• Add FAQ for contributors which can be updated easily with new info. EG: Should contributors @ mention full time Gitlabbers if so who?

Tasks created in original plan that are outstanding:

• Add bullet points on the Get started!docs page
• Move Architecture of GitLab to the bottom of getting started or a new section

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Relevant files and pages:

https://gitlab.com/gitlab-org/gitlab-ce/tree/master/doc/development/contributing

https://docs.gitlab.com/ee/development/contributing/
https://docs.gitlab.com/ee/development/contributing/design.html
https://docs.gitlab.com/ee/development/contributing/issue_workflow.html
https://docs.gitlab.com/ee/development/contributing/merge_request_workflow.html