feat: version documentation

Created by: ashleythedeveloper

This PR releases the existing content within the UNTP Specification website as version v0.6.0. Versions will now be accessible within the version dropdown in the navbar.

I've also placed static assets that are likely to change within the /docs directory, such that they will be versioned when a documentation release occurs.

I've also moved the global assets referenced within the site to the static folder as per the documentation.

Note "Work in Progress" contains the latest docs that have not yet been released.

Versioning