Reshape top half of BBM page, reorder, fix subheads

What does this MR do?

Continues work on https://docs.gitlab.com/ee/update/background_migrations.html to bring it closer to current spec. This revision focuses on the top of half of the page:

• Move the old-style, non-batched migration info down to the bottom of the page so we show the info about the current (batched) method first
• Move the version history info for BBM to the top of the page, and rewrite slightly so it's clear what we're talking about
• Create an umbrella subheading (h3) for "advanced features"
• For the advanced features, we no longer need separate subheadings for "enable this", so let's cut down on the number of subheadings.
• Add in FLAG boxes where needed
• Rebuild version history notes to match current style. Normally we'd take out version history notes from 13.x, but they're actually still needed here, so let's leave them for another year.

This merge request does NOT tackle anything in the very long troubleshooting section. After I get the top half of the page into its final + clean form, I can go through the troubleshooting items and pull out the ones that should become tasks.

Related issues

• Related to Retry failed batched background migrations (!124821 - merged) where I initially realized this page needed a LOT of work
• First round of structural work was done in Begin structural fixes and renamings on BBM upd... (!125166 - merged)
• Related to Revises top section of background migration page (!125178 - merged) which started revising the top half of the page, but stayed away from the feature flag subheadings because those changes would need both style review AND technical review.

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.