Revise first third of Suggestions page

What does this MR do?

As I expected in Mention icon for multi-line suggestions (#390425 - closed), I can't fix the "multi-line suggestions" section of https://docs.gitlab.com/ee/user/project/merge_requests/reviews/suggestions.html without fixing everything that comes before it on the page.

This merge request tackles the first third of the page, which hasn't been touched since this page was hacked out of a larger, more digressive page:

• Creates subheadings for adding and applying suggestions
• Fixes errors in the instructions
• Removes images from 13.x
• Uses SVG icons to to reference UI elements when appropriate / possible
• Rewrites for flow, clarity, common stinkin' sense, profit
• Acknowledges the existence of batched suggestions
• Leaves a fairly obvious place to blend in instructions about multi-line suggestions
• Tries really hard to not boil the ocean by leaving the rest of the page for the second pass

Related issues

Related to Mention icon for multi-line suggestions (#390425 - closed) but does not close it

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.