Full CTRT revamp of merge trains page

What does this MR do?

This MR revamps the Merge Trains documentation:

• Copy edits the whole page.
• Removes screenshots that aren't needed thanks to improved language.
• Makes sure all sections follow CTRT standards.
• The most important changes are the simplifications and clarity improvements. Merge trains are complicated and often confusing, but a great feature that drives people to Premium, so it's important to make this feature easier to understand.

The most difficult section to deal with was the intro and example, which has a lot of conceptual information. I decided to trim the intro as much as I could, and move the more difficult concepts to the example section, now titled Merge train workflow, as recommended at https://docs.gitlab.com/ee/development/documentation/topic_types/concept.html#concept-headings

Before / After

Readability: 8.69 -> 7.87

Before	After

Related issues

• Resolves #378236 (closed)
• Related to #217441 (closed)

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 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.