Adding Same Content across Different Pages

I'm trying to add a custom sidebar that only shows on specific pages, which uses a Javascript Markdown drop-down menu.  It'd be a custom navigation panel for custom shop pages I'm building.  Is there any way to "write" content and hide it in the header, and then add a script to the sidebar that "pulls" this content into it?  The goal is to write the menu once and for it to show on every relevant page, which would also allow me to edit it once if it ever needs editing.

Thanks!