Make it easy to copy versioned URL

[rwinch]

It is difficult to include a link that includes the version to the latest version of documentation. This is because

• The URL to latest version of the documentation does not include a version
• If the versioned URL to the latest version of the documentation is used, it redirects to the URL with no version

We should include a simple way to copy the versioned URL

Related spring-projects/spring-boot#47965

cc @ciis0

[rwinch]

This is now released in antora-ui-spring 0.4.21. User's can copy the versioned URL from the breadcrumbs with the URL icon. An example can be see from Spring Security's reference (you will need to refresh the browser to pick up the updated CSS). You can find a screenshot below:

Clicking on the icon will add the versioned URL to the clipboard and briefly change the icon to a clipboard to indicate that it was copied.