[mkdocs] - ensure all codpen example use same plotly.js version (and not plotly-latest)

[LiamConnors]

Some codepen examples use plotly-latest and then show a warning in the console

This is the case in our existing docs too, but while we are doing the migration it would be a great time to tidy this up and ensure all examples always use the same version - a specific version like 3.3.0 for example