tweak some website layout and style settings

- make previous/next buttoms smaller and show on top and bottom of page
- make tab header items smaller and more rounded
- reduce padding on a few selected items to make pages more compact

remove obsolete lammps.css file variant

doc/utils/sphinx-config/_static/css/lammps.css

@@ -18,3 +18,30 @@
 .versionmodified {
     font-weight: bold;
 }
+
+hr {
+     margin: 12px 0;
+}
+
+.rst-content {
+     margin-bottom: 12px !important;
+}
+
+.ui.tabular.menu .item {
+    padding-right: 1em;
+    padding-left: 1em;
+    padding-top: 0.1em;
+    padding-bottom: 0.1em;
+    border-radius: 6px;
+}
+
+.btn {
+    padding: 4px;
+    font-size: 80%;
+    border-radius: 4px;
+}
+
+.rst-breadcrumbs-buttons {
+    margin: 0;
+}
+