Update toolbar customisation docs with button reordering info
Per discussion with @mcrha on IRC, by default the ordering of toolbar buttons in .ui files (in $PREFIX/share/evolution/ui/ and $HOME/.config/evolution/ui/) is ignored.
This can be worked around by setting the position='top'
attribute on toolitem
etc. (see https://gnome.pages.gitlab.gnome.org/gtk/gtk3/class.UIManager.html). It also works on non-empty placeholder
elements despite the GTK documentation. This will move the toolbar items before all others that don't have the attribute set (and so can be set on all items in a group to enable ordering), and ordering between items with the attribute set is respected, although it may be in reverse order.