[mkdocs] - some default values are not correct

[LiamConnors]

The are some cases of "True" given instead of "true". Seems to always be on the visible property

/javascript/reference/scatter/#scatter-visible

/javascript/reference/pie/#pie-visible

[raman976]

Hey @LiamConnors please assign me this issue. I would like to work on this.

[robertclaus]

@raman976 unfortunately this issue isn't a great fit for first time contributors since it's part of a larger internal rewrite. We would love to get your help on other issues in the repo though!