Adds a base domain doc section on cluster pages

Note: This MR should be merge after https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/24580 is merged.

What does this MR do?

Adds a base domain doc section on cluster pages. The changes are included for project and group cluster documentation pages. Also specifies that AUTO_DEVOPS_DOMAIN will be deprecated on 12.0

Related issues

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/52363 Closes

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• [-] Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards