My current understanding is that Weebly Forums is built using the WordPress CMS which has a lot more features & widgets that Weebly doesn’t have.
I’m not sure if some html or script could be used to build the (site structure) list? There would need to be a way of accessing the web site structure (which must be stored somewhere in the Weebly CMS). The web pages themselves are not stored in a structured way, but all at the top level.
There is no automatic way of building breadcrumbs.
If it is absolutely necessary, you will have to do it manually. Add Text and link each piece to a corresponding page.
Like Paul mentioned, there is no hierarchy. So, you may actually end up confusing your visitors instead of helping them navigate better. If I were you, I would define an intuitive navigation from Menus, and if the need demands, I would create a static header region to cater to any special purposes.
If you can specify what you are trying to accomplish, that will help with suggesting better alternatives (if any).