[Backend] Add ability to link to a new section within the parts-kit

[ekfuhrmann]

Similar to #6, we could maybe leverage postMessage() to update the current page of the parts-kit in case you would like to cross-reference another section.

As a temporary workaround, you can use <a target="_parent" href="<URL>">link</a> or just have the cross-reference open in a new tab.