Create email templates for inviting expert speakers

[yochannah]

@Camille-Gonz-VA you already do the invites. Could you please use hackmd.io to create a markdown template for emails, so we can generate them automatically, and put it in this repo in an appropriate folder?

lmk if you have any questions. Once the template is drafted, @bebatut and/or @NPDebs will help create the script to generate emails (so we never have to manually add timezones).