DOC: increased width of text area (#4159)

See #4159

This adds a minimum and maximum width of the text area (instead of one fixed value of, at the moment 600px), and I also increased the maximum value (from 600 to 840). 

I took the values of the standard sphinx docs which are 780-1080 (minus 240 for the sidebar, in the standard sphinx docs the width is defined for the whole body, here only for the main text area)