Copy-paste the following into a file named "sidebar.js":

var html = '<a href="index.html"><h3>Home</h3></a>';

var section1 = ['HTML', 'Java', 'CSS']//write one-word link names in this list
html += '<h3>Demo:</h3>';
for (i = 0; i<section1.length; i++){
	html += '<div class="navlink"><a href="' + section1[i] + '.html">' + section1[i] + '</a></div>'

document.getElementById("templaterr-sidebar").innerHTML = html;

To add/remove/edit links, change the items in the list section1[].
Updates will apply to all pages.

The links must be one word, and capitalization counts, because Link1 will lead to Link1.html, but link1 will lead to link1.html. For multi-word links, external links, or links where the link text doesn't match the name of the page it links to, use this line instead:

html += '<div class="navlink"><a href="">LINKTEXT</a></div>'

and replace the youtube link with the link of your choice.

You can add images and anything else you want, too:

html += '<a href="">
	<img src=""></a>'