Improve add cluster documentation

What does this MR do?

Improved the experience of adding clusters in GitLab by:

• Centralizing requirements information.
• Updating procedures for current state of the product.
• Streamlining some information.

The bulk of the changes are on the Adding and removing Kubernetes clusters page:

• Before: https://docs.gitlab.com/ee/user/project/clusters/add_remove_clusters.html
• After: http://docs-preview-ee-21806.178.62.207.141.xip.io/ee/user/project/clusters/add_remove_clusters.html

Closes: #30575 (closed).

Relates to: gitlab-design#692 (closed).

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

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.

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.