Copy edit support statement to differentiate target audiences

What does this MR do?

As highlighted in the related issue, and the discussions in the MR that prompted the issue (!141919 (comment 1735178986)), we are mixing messaging on this important page now.

We need to separate the content that is targeting external users interested in trying out experiment/beta features, from the content that is targeting internal team members developing/planning features.

• Before: https://docs.gitlab.com/ee/policy/experiment-beta-support.html
• After: https://main-ee-144894.docs.gitlab-review.app/ee/policy/experiment-beta-support.html

This MR:

• Creates a new ## GitLab Experiment and Beta development guidelines section to contain all the internally targeted information. As part of this:
  • Three new sections were created for Experiment/Beta/GA information that is internally facing, moved out from the bullet list of user-facing details higher up in the page.
  • Review criteria details are moved into these three sections as well.
• Reorders the sentences in the ### Provide earlier access section, from one large paragraph into three with more logical flow.
• Moves Sid's ## All features are in production section up a little higher in the page, but removes the duplicated language.
• "Standard" TW copy edits throughout, including simplifying/clarifying information, splitting long lines over multiple lines for easier maintainability, etc.

Related issues

Resolves #439560 (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 a new page, add the product tier badge 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.