Add example for using API to transfer project

What does this MR do?

This MR updates the documentation on transferring a project to a new namespace to include an example. This MR was motivated by the issue:

Clarify "Transfer Project to Group" API endpoint example

I have extended the documentation to include a sample cURL command that demonstrates how to transfer a project to a group (or subgroup) by specifying the numeric ID of the target namespace.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

documentation