Navigation Documentation: Add wc-admin doc

[psealock]

Continuing with #5421 to create a working example for 3PD to register into the Navigation, we'll need some documentation so that plugins can easily integrate.

Deliverables

• Create a WC Admin README or doc.
• Explain the significance of the Navigation project.
• Provide step by step instructions on registering PHP as well as JS custom links and why that is useful for router-aware links.

cc @pmcpinto

[pmcpinto]

Also cc @loranallensmith so we can start collaborating on this

[psealock]

As part of this issue, a README added to the example plugin would be helpful.
https://github.com/woocommerce/woocommerce-admin/tree/main/docs/examples/extensions

[psealock]

A note to those following this issue, I split it into two: This issue to track documentation in wc-admin and #5477 to track outbound communication with third party developers.