Doc update needed in 'An example custom template' section

[marckassay]

This section, 'An example custom template', is referring to, I believe, previous generation of Jinja templates. Which is not aligned with what I'm seeing here in master: share/jupyter/voila/templates.

Additional updates are needed in 'Creating your own template' section.

[bmorris3]

👍🏻 This would be handy for those of us maintaining custom templates like jdaviz.