New doc design: Sphinx Bootstrap 4 Theme

Now we use an own, mobile-first, responsive sphinx theme based on Bootstrap 4.
It makes navigation on the very long "Python module instaloader" page a lot
easier. Also, it looks better.

This solves all the problems we had with sphinx-bootstrap-theme.

docs/_static/instaloader.js

@@ -0,0 +1,22 @@
+$(function () {
+    $('[data-toggle="tooltip"]').tooltip();
+
+    $('.doc-sidebar > ul > li.current').attr("id", "localtoc");
+    $('#localtoc ul').addClass("nav flex-column");
+    $('#localtoc ul li').addClass("nav-item");
+    $('#localtoc ul li a').addClass("nav-link");
+    $('.doc-content').scrollspy({target: '#localtoc'});
+
+    const top_href = '#' + $('.section:first').attr("id");
+    $('#localtoc > a.current').attr("href", top_href);
+
+    $('#navbarToc a').on("click", function () {
+        const href = $(this).attr("href");
+        if (href === '#') {
+            window.location.href = top_href;
+        } else {
+            window.location.href = href;
+        }
+        $('#navbarToc').modal('hide');
+    });
+});