Adds How-it-works section to BBM

What does this MR do?

This is the first part of the efforts to re-structure the Batched Background Migrations docs https://docs.gitlab.com/ee/development/database/batched_background_migrations.html

This MR adds How it works? section to the docs, and:

• Adds a high level explanation of how BBM works;
• Adds Execution mechanism sub-section; Describe the execution mechanism behind BBM, with a diagram;
• Adds Migration optimization sub-section; Describe how the migrations are optimizes during it's execution;
• Moves the Idempotence section to a sub-section, under How it works?;
• Moves the Job retry mechanism section to a sub-section, under How it works?;
• Moves the Failed batched background migrations section to a sub-section, under How it works?;
• Moves the Throttling batched migrations section to a sub-section, under How it works?; This docs were not merged yet. See: !119190 (merged)

Author's checklist

• Depends of !119190 (merged)
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

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.

#388789 (closed)