docs: add divider using CSS

instead of hardwiring the formatting in the html code, do this using
CSS, more flexible this way.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

docs/_static/css/custom.css

@@ -32,3 +32,7 @@ hr {
 dl dt:hover > a.headerlink {
     visibility: visible;
 }
+
+dl.confval {
+    border-bottom: 1px solid #cacaca;
+}

docs/_templates/db_option.tmpl

@@ -5,7 +5,3 @@
    {% if type %}* **Type:** ``{{ type }}``{% endif %}
    {% if default %}* **Default value:** ``{{ default }}``{% endif %}
    {% if liveness %}* **Liveness** :term:`* <Liveness>` **:** ``{{ liveness }}``{% endif %}
-
-.. raw:: html
-
-   <hr/>