Provide more specifics on formatting code blocks in docs

What does this MR do?

Code blocks containing shell commands need additional explanation to ensure syntax highlighting displays properly. Shell commands WITH output need a $ before the command, and a blank line before the output. Shell commands WITHOUT output should not have a $ before the command.

cc @marcel.amirault and @axil.

Related issues

Closes #199675 (closed) Related to gitlab-docs#633 (closed) about preventing $ in commands from being copied.

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.