Update examples/docs to use more robust embed_version

[jlstevens]

Using the simple change in holoviz/holoviews#2532 embed_version can be made robust so that you could install from a tar.gz file in a clean environment without an internet connection (for instance).

[ceball]

And add a version of the test where e.g. there's no access to remote autover url, so that embed_version() fails.