Default beginAtZero to true for linear axes

[etimberg]

Resolves #5939

[benmccann]

grep -r beginAtZero docs/ - there's a handful of docs that can be updated. E.g. mentioning the default value for beginAtZero and some that specify beginAtZero: true which is no longer required

[benmccann]

There's a handful of samples where it'd no longer be required as well

[benmccann]

Also, I've been thinking that rather than setting this for the linear scale generally we should set it just for bar charts. If you look at the two articles in the original bug report they give a good explanation of why it's desired for bar charts and not line charts. Also, one of the issues this fixes is that the lowest bar disappears on the bar chart by default, but that's only an issue for bars

[etimberg]

I’d be open to just doing it for bar charts. That would revert most of these changes. I’ll close this and open another PR

[kurkle]

I vote for the "just for bar charts"