Improve image size and proportions

[wlangera]

Currently, default knitr options knitr::opts_knit$set are used for image size and proportions (code line).
This causes the layout to be messed up in some cases.

proportion

Below is an example where the different error bars are not well separated and the legend is cut off on the documentation website, but the image looks fine on the pkgdown website:

pkdown

documentation website

size

Also image size is rather small.
If you zoom in on an image from the software, there is no zoom (see here).

But images from the documentation website itself are much larger, e.g. b3verse guide:

[peterdesmet]

@sannegovaert can you see if we can control this knitr options in b3doc?