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.css

@@ -0,0 +1,74 @@
+@import url(bootstrap-4.1.3.min.css);
+a {
+  color: #008d06; }
+  a:hover {
+    color: #f48400; }
+
+code {
+  color: #212529; }
+
+.highlight {
+  border-radius: 0.3em; }
+  .highlight pre {
+    padding: 0.7em;
+    color: #fff; }
+  .highlight .c1 {
+    color: #008d06; }
+  .highlight .kn {
+    color: #f48400; }
+  .highlight .s1, .highlight .s2, .highlight .mi {
+    color: #ff7efd; }
+
+.doc-content {
+  top: 4.0625em;
+  position: relative;
+  height: calc(100vh - 4.0625em);
+  overflow-y: auto; }
+
+.doc-sidebar {
+  position: fixed;
+  padding-top: 0.5em;
+  top: 3.5625em;
+  bottom: 0;
+  right: 0;
+  overflow-y: auto; }
+  .doc-sidebar ul {
+    padding-left: 1em; }
+    .doc-sidebar ul.current {
+      list-style-type: none;
+      padding-left: 0; }
+  .doc-sidebar li {
+    padding-top: 0.2em;
+    padding-bottom: 0.2em; }
+    .doc-sidebar li a {
+      color: #008d06; }
+      .doc-sidebar li a.current, .doc-sidebar li a.active {
+        color: #f48400;
+        font-weight: 500; }
+      .doc-sidebar li a:hover {
+        color: #f48400;
+        text-decoration: none; }
+
+.admonition {
+  border-radius: 0.3em;
+  border: 1px solid #ccc;
+  background-color: #f8f9fa; }
+  .admonition .admonition-title {
+    color: #ff7efd; }
+
+.topic {
+  border-radius: 0.3em;
+  background-color: #f8f9fa; }
+
+.nav-link {
+  padding-top: 0;
+  padding-bottom: 0; }
+
+.btn-success {
+  background-color: #008d06;
+  border-color: #008d06; }
+  .btn-success:hover {
+    background-color: #f48400;
+    border-color: #f48400; }
+
+/*# sourceMappingURL=instaloader.css.map */