Yes, you can. The heading.icons value can be defined as a function that returns a string, that returned value is what we'll render for the icon. In that function you have access to a sections property which is a list of integers indicating that index information.

Based on your example here's what you would do:

require('render-markdown').setup({
 heading = {
 icons = function(ctx)
 return table.concat(ctx.sections, '.') .. ' '
 end,
 },
})

You're free to change this however you feel, maybe adding an icon in some cases, changing how the values are joined together, changing the suffix, it's all completely up to you.