docs: adapt TOC styling to something a bit more light weight

marketing noted that it looked quite heavy weight, to the point where
it was hard to read to the full black squares, bold monospace text
and bottom borders with rather distinctive darkness.

Address those by:
* change color for list points from black to mid-dark grey
* use empty circles for second heading level
* ensure monospaced text has a normal font weight in the TOC headings
* some lighter color for the bottom border

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

docs/custom.css

@@ -25,6 +25,23 @@ ul li.toctree-l1 > a {
     color: #000;
 }
 
+div.sphinxsidebar ul {
+    color: #444;
+}
+div.sphinxsidebar ul ul {
+    list-style: circle;
+}
+div.sphinxsidebar ul ul ul {
+    list-style: square;
+}
+
+div.sphinxsidebar ul a code {
+    font-weight: normal;
+}
+div.sphinxsidebar ul ul a {
+    border-bottom: 1px dotted #CCC;
+}
+
 div.sphinxsidebar form.search {
     margin-bottom: 5px;
 }