Fix for docments table column widths

[simonkeys]

Because Markdown tables must have fixed-width columns, the layout of the docments table is less than ideal. Here's an example from the nbdev documentation:

We can use a css rule in styles.css to get auto spacing back:

.table colgroup col {width: auto !important;}

That looks a lot nicer to me:

If it's too heavy-handed to disable all colgroup width specifications, maybe we could add a custom class to the docment table and use that in the css rule?