improve admonition icons
improve admonition icons


Former-commit-id: 63cd6c8828afe1744524695198636a5c9f8f0832 [formerly dd8c67b6189625a23e23825f3b4e216010cd283a]
Former-commit-id: 6d0ec20f010595324cce40e714b1559e39080c33
LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io,LibraryCarpentry/librarycarpentry.github.io