Add ability to specify number of columns in the section navigation tiles

[bedroesb]

• col: give an integer to specify the number of columns/section cards per row. Default: 2.

Example:

{% include section-navigation-tiles.html type="example_pages" affiliations=true search=true except="index.md" col=3 %}

or for the simple navigation tiles:

{% include section-navigation-tiles-simple.html type="example_pages" col=3 %}

IMPORTANT. The columns in contributor tiles are from now on specified with the col parameter, similar to the section navigation tiles.