Add support for toctree captions

I do not know if that was supported with doctrine/rst-parser, but there
is such a thing as a caption option for the toctree directive, and it
seems that phpDocumentor/guides supports it.
When I tried it, it produced a "p" element with a caption class.
Adding support for that means we could get rid of the toc and tocheader
directly.
These custom directives appear to be the directives that cause the most
unfixable warnings (unfixable without a custom guides extension, that
is).
We might end up with an extension anyway if there are other warnings,
but I think this could be a nice simplification.

source/styles/index.scss

@@ -123,6 +123,14 @@ h6 {
   color: #fff;
 }
 
+.sidebar-sticky .toc .caption {
+  padding: 8px;
+  font-size: 20px;
+  background-color: $doctrine-dark-blue;
+  margin-bottom: 0;
+  color: #fff;
+}
+
 /** 1st Level */
 .sidebar-sticky .toc ul {
   list-style-type: none;