how to add icon(s) for markdown headers?

[hopezh]

May I ask if there is a way to customize icons for markdown headers?

The following is what outline.nvim is showing:

[hedyhli]

Hello, this is nvim-navbuddy.

To configure icons for markdown in outline.nvim, you configure it for outline.nvim. Take a look at this recipe example in the readme:
https://github.com/hedyhli/outline.nvim?tab=readme-ov-file#custom-icons

You can use a symbols.icon_fetcher function to override icons specifically for markdown filetypes.

    symbols = {
      icon_fetcher = function(kind, buf)
        local ft = vim.api.nvim_buf_get_option(buf, "ft")
        if ft == 'markdown' and k == 'String' then
          return ""
        end
        return false -- Fall back to `symbols.icon` table, or `icon_source`
      end,
      icon_source = "lspkind",
    }

In this example I have disabled icons entirely, for markdown filetypes.