fix: Resize the column from just-the-docs theme

[SensibleWood]

The security page seems to have uncovered a usability issue with the site, namely the centre column is fixed width and therefore makes the security diagram look tiny (even on a huge monitor with maximised window):

It'd be good to resize this centre column so as make such diagrams clearerer without having to zoom in.

[lornajane]

I am not sure I agree that this should be changed. Could we revisit the diagram layout on this one? The line length seems about right for me in terms of readability in proportion to the font size. The majority of the pages do not have diagrams.

[SensibleWood]

There's also this one btw:

That one already existed and is quite small without zooming

[SensibleWood]

The majority of the pages do not have diagrams.

6 of the 8 pages have diagrams btw