Refine upgrade instructions for installation methods

What does this MR do?

For: #555861, this MR refines the instructions for many of the installation type-based upgrade documentation. The pages should flow in a logical sequence:

1. From landing page (https://docs.gitlab.com/update/).
2. To "before you upgrade" page (https://docs.gitlab.com/upstream-review-mr-ee-209230/update/plan_your_upgrade/).
3. To "generic upgrade GitLab" page (https://docs.gitlab.com/upstream-review-mr-ee-209230/update/upgrade/).
4. To a "specific upgrade GitLab page".

Apart from reminding readers that they need to perform some tasks before they upgrade, they shouldn't have to go back and forth. They should only have to go forward.

Preview of changes

Before you upgrade:

• Before: https://docs.gitlab.com/update/plan_your_upgrade/
• After: https://docs.gitlab.com/upstream-review-mr-ee-209230/update/plan_your_upgrade/

Upgrade a GitLab instance (generic page):

• Before: https://docs.gitlab.com/update/upgrade/
• After: https://docs.gitlab.com/upstream-review-mr-ee-209230/update/upgrade/

Upgrade Linux package instances (specific page):

• Before: https://docs.gitlab.com/update/package/
• After: https://docs.gitlab.com/upstream-review-mr-ee-209230/update/package/

Upgrade a Docker instance (specific page):

• Before: https://docs.gitlab.com/update/docker/
• After: https://docs.gitlab.com/upstream-review-mr-ee-209230/update/docker/

Upgrade a self-compiled instance (specific page):

• Before: https://docs.gitlab.com/update/upgrading_from_source/
• After: https://docs.gitlab.com/upstream-review-mr-ee-209230/update/upgrading_from_source/

The specific pages for GitLab Helm chart instances and GitLab instances with Operator must be done in separate MRs.

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding a new page, add the product availability details under the H1 topic title.
• 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.