Clarify feature flag docs

What does this MR do?

I had some trouble understanding the feature flag docs, and felt that some small changes might make these a little easier to understand. The content should be mostly unchanged, but with additional details and clarifications.

Some of the changes:

• Change some sub-sections for the creation instructions to be bulleted, because they do not have to be done in order.
• Reorder the sections a bit, change the nesting.
• Split the enable/disable instructions to their own section, that can then be removed later when the feature flag is gone (and it doesn't intrude on the explanations from the feature as much anymore)
• Call the old rollout strategy "legacy" (this might not be popular, but it makes it clear it's the old style).
• Make the Feature Flags Strategies details much clearer, by explaining how to add/change strategies, list the available strategies, make it clear which sections are specifically strategy-related, and generally expand the details.

Related issues

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have developer access or higher (for example, GitLab team members or Core Team members)
  • Apply the documentation label, plus:
    • The corresponding DevOps stage and group label, if applicable.
    • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
    • development guidelines and Documentation guidelines when changing docs under development/documentation/*.
    • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
  • Assign the designated Technical Writer.

Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Add GitLab's version history note(s).
• Add the product tier badge.
• Add/update the feature flag section.
• If you're changing document headings, search doc/*, app/views/*, and ee/app/views/* for old headings replacing with the new ones to avoid broken anchors.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
  • Add Technical Writing and docs:: workflow label.
  • Add docs-only when the only files changed are under doc/*.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.