Add docs to help choose the best GitLab installation method

What does this MR do?

Adds documentation to help customers choose the best installation method.

Related issues

Closes #54308 (closed)

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
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8646

Review checklist

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