Zero Downtime upgrades documentation rewrite

What does this MR do?

MR includes a much needed top to bottom rewrite of the Zero Downtime upgrades documentation to be up to date and follow latest processes and recommendations as follows:

• A full rewrite of the Zero Downtime process to be up to date with streamlining throughout to improve readability and have a clear order
• A new Before you Start section to help set expectations of the general process, what it involves and the general challenges of Zero Downtime upgrades for any distributed application
• A new Requirements and considerations section that details more things to consider before doing the process
• A clear upgrade order has now been added
• Better handling of existing connections by adding guidance to drain Rails nodes gracefully with one built in process offered as an example
• Significant streamlining of Geo section to refer to already existing docs and reduce duplication

The docs have been streamlined notably as part of this rewrite to improve readability and include some new guidance on how to ensure graceful handling of existing connections where possible.

Related issues

Closes #326519 (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 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.