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You're using 'use strict'; in all functions except the top one. Why not declare it at the top of your file?

You seem to have thought about readability. You're declaring variables for just that purpose here:

    let currentLink = exampleLinks[i];
    let currentMenuContainer = exampleMenus[i];


So why oh why do I see magic numbers in your code?

setTimeout(addLinks, 2000);
let topicLink = document.querySelectorAll("a.doc-topic-link")[0];
topicLink.href.split('/')[5]
currentLink.href.split('/')[7]
menu.insertBefore(elem, menu.children[0]);


Of those 5, arguably the first and latter are acceptable. The 3rd and 4th are the most magical of the lot. You explained the first in a comment, but I'd personally move it to a variable.