Clarify and track removal of deprecation messages and features

What does this MR do?

• Creates an Issue template for removing deprecation messages.
• Clarifies when and how deprecations messages should be removed, and why it's like that.
• Frames the documentation in a more structured format.
• It informs users by other important docs to learn about:
  • When should we deprecate:
    • Deprecate and announce at latest 3 milestones before the major milestones planed as removal date.
  • The Omnibus deprecation policy, which explains:
    • We should announce in all required ways: (blog post, omnibus warning, deprecation page).
    • The correct removal date depending on the deprecation classification (regular vs sensitive).
• Adds some newlines that don't impact on the output for the markdown but help with maintainability.
• Asks authors to create a deprecation issue and a removal issue ahead of time so that we don't forget to execute these steps and be better prepared to plan our milestones.

Related issues

Closes #7807 (closed)

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

If you are only adding documentation, do not add any of the following labels:

• ~"feature"
• ~"frontend"
• ~"backend"
• ~"bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Review 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 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.
  • 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 above reviews. Maintainer's review can occur before or after a technical writer review.
• Ensure a release milestone is set.