Draft: Improve GitLab Dedicated maintenance docs

What does this MR do?

Aims to make the details of GitLab Dedicated's maintenance and release upgrade schedule easier to find through both search and navigation:

• Moves the maintenance and release rollout docs to a separate page GitLab Dedicated maintenance and release schedule
• Edits the content to make it easier to scan and make some style guide fixes

View the page in the review app here: GitLab Dedicated maintenance and release schedule

What prompted this?

I was searching for customer-facing docs on GitLab Dedicated's release schedule and I could not find anything through search (docs searchbar or google search). Only while reviewing the open ~"documentation" issues I found the documentation here. That page had all the details laid out clearly, just not where I expected to find them (bottom of the create instance page).

These docs were originally added with this MR: Doc: Adds GitLab release rollout schedule for G... (!145491 - merged))

Related issues

https://gitlab.com/gitlab-com/gl-infra/gitlab-dedicated/team/-/issues/4166+

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.