Increase width of doc

[adehecq]

Currently, the width of the documentation is too narrow compared to our standard code width (120 characters). This means the code does not appear all at once and one needs to scroll horizontally.
There are some solutions here that can be tested.
The comment by Yahya Yahyaoui seems the simplest, so worth testing.

[rhugonnet]

To leave a trace for later:
Tested the html_theme_options = {'body_max_width': '70%'}, does not work because we use the html_theme = 'sphinx_rtd_theme' theme.
We could only change the width, but maybe it's worth discussing potentially changing theme entirely!