Docs: review/update "Create MRs" documentation

Problem to solve

From gitlab-design#441 (closed), "Create a Merge Request" is one of the JTBD made by UX folks.

The docs team can help by refactoring and updating the "Create MR" documentation. Some of it was already done.

Proposal

Address the following:

• Consolidate information in one SSoT
• Describe the different forms of creating an MR
• Improve the content
• Remove redundancies (continuing &1280 (closed))
• Present reasons for shipping smaller MRs

(To be adjusted as needed).

Who can address the issue

@marcia as Q4/2019's OKR

Other links/references

• gitlab-design#692 (closed)
• #30646 (closed)
• gitlab-design#444 (closed)

Related documentation:

• https://docs.gitlab.com/ee/user/project/merge_requests/#creating-merge-requests
• https://docs.gitlab.com/ee/user/project/merge_requests/creating_merge_requests.html
• https://docs.gitlab.com/ee/gitlab-basics/add-merge-request.html#how-to-create-a-merge-request
• https://docs.gitlab.com/ee/development/contributing/merge_request_workflow.html