Combine "Product availability details" and "Document product versions""

What does this MR do?

Product availability details and Document product versions both outline the same style standards in the style guide. This MR consolidates the information in both to improve docs navigation.

• Adds content from Document product versions to Product availability details
• Adds a redirect to versions.md.
• Creates a new page for Deprecations and removals. Although deprecations and removals are related to availability details, there's a separate process for removing pages and topics that doesn't quite fit in with Product availability details.
• Edits for headings and content to improve conciseness and maintain parallelism with other topics in `availability_details.md`.

Combining Experiment and beta features into Product availability details will be addressed in a follow-up MR.

Click for new topic locations

Topic	New location
View older documentation versions	backporting.md
Add a history item	availability_details.md#version-history
Documenting updates to a feature	availability_details.md#updated-features
Making features available as part of a program	availability_details.md#features-made-available-as-part-of-a-program
Moving subscription tiers	availability_details.md#moved-subscription-tiers
Changing the feature status	availability_details.md#changed-feature-status
Features introduced be behind feature flags	availability_details.md#features-behind-feature-flags
Inline history text	availability_details.md#inline-history-text
Deprecations and removals (entire section)	deprecations_and_removals.md (new page)
Which versions are removed	availability_details.md#removing-versions
Timing version removals	availability_details.html#timing-version-removals
Promising features in future versions	availability_details.md#promising-features-in-future-versions

New nav

With gitlab-docs!4950 (merged), this will be the new nav:

Related issues

#469579

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.