Add documentation for /ship quick action

What does this MR do and why?

This MR adds documentation for the /ship quick action that was introduced in !78998 (merged).

The /ship quick action is an experimental feature that:

• Creates a merge request pipeline
• Sets auto-merge on the merge request

This streamlines the workflow by combining two common actions into a single command.

Documentation details

The documentation:

• Specifies that this is an experimental feature
• Notes that it's behind the ship_mr_quick_action feature flag
• Indicates the feature flag is disabled by default
• States it was introduced in GitLab 18.6
• Links to the original MR (!78998 (merged))

Related to !78998 (merged)

Related to #210538