Add a warning label to the example implementation

[larsvilhuber]

• the example at https://transparency-certified.github.io/trace-specification/docs/sample-implementation/ was done with a pre-release version of TRACE/TROV.
• add a banner ("warning") at the top of the page that states that. References within the page may not point to current (versioned) pages.
• An update to the example is pending.