docs: internals: columnize rather long ToC

docs/borg_theme/css/borg.css

@@ -28,3 +28,16 @@
 #usage dl dl dd {
     margin-bottom: 0.5em;
 }
+
+#internals .toctree-wrapper > ul {
+    column-count: 3;
+}
+
+#internals .toctree-wrapper > ul > li {
+    display: inline-block;
+    font-weight: bold;
+}
+
+#internals .toctree-wrapper > ul > li > ul {
+    font-weight: normal;
+}

docs/internals.rst

@@ -39,7 +39,7 @@ hash-table of all chunks that already exist.
     (Repository) or remotely (RemoteRepository).
 
 .. toctree::
-    :caption: Contents
+    :caption: Internals contents
 
     internals/security
     internals/data-structures