Begin updating the content of the create-MR page

What does this MR do?

Begins the technical review of https://docs.gitlab.com/ee/user/project/merge_requests/creating_merge_requests.html. This page is #5 on the list of most viewed pages for devopscreate. (From internal-only spreadsheet.) I knew from a recent merge request from a community contributor that multiple subheadings needed updates to steps.

This MR completes updates to several subheadings on the page, but not all of them, because I'm starting to run out of steam. It updates/corrects:

• From the merge request list
• From an issue
• When you add, edit, or upload a file
• When you use Git commands locally
• When you create a branch
• When you work in a fork
• By sending an email
• Add attachments when creating a merge request by email
• Set the default target project

Related issues

• Related to https://gitlab.com/gitlab-org/technical-writing/-/issues/936+
• Related to Updated creating MR tasks (!136782 - closed) where I learned the fork-related steps on this page needed work, and others likely did as well

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.