Add rendering of \{sub}section

\section, \subsection and \subsubsection are commands that can be added
to \page for creating multi-level section rendering, including a title
and content.

https://www.doxygen.nl/manual/commands.html#cmdsection
https://www.doxygen.nl/manual/commands.html#cmdsubsection
https://www.doxygen.nl/manual/commands.html#cmdsubsubsection

Implement the rendering using Docutils nodes.title() which already uses
the structure of the document to attribute a proper heading level, which
should not need extra handling since the Doxygen XML already guarantees
the proper structure.

Signed-off-by: Fabio Utzig <utzig@apache.org>

breathe/renderer/sphinxrenderer.py

@@ -1344,8 +1344,19 @@ def visit_docmarkup(self, node) -> List[Node]:
             print("Warning: does not currently handle 'small' text display")
         return [creator("", "", *nodelist)]
 
-    def visit_docsect1(self, node) -> List[Node]:
-        return []
+    def visit_docsectX(self, node) -> List[Node]:
+        '''
+        Docutils titles are defined by their level inside the document so
+        the proper structure is only guaranteed by the Doxygen XML.
+
+        Doxygen command mapping to XML element name:
+        @section == sect1, @subsection == sect2, @subsubsection == sect3
+        '''
+        section = nodes.section()
+        section['ids'].append(node.id)
+        section += nodes.title(node.title, node.title)
+        section += self.render_iterable(node.content_)
+        return [section]
 
     def visit_docsimplesect(self, node) -> List[Node]:
         """Other Type documentation such as Warning, Note, Returns, etc"""
@@ -1986,7 +1997,9 @@ def dispatch_memberdef(self, node) -> List[Node]:
         "docimage": visit_docimage,
         "docurllink": visit_docurllink,
         "docmarkup": visit_docmarkup,
-        "docsect1": visit_docsect1,
+        "docsect1": visit_docsectX,
+        "docsect2": visit_docsectX,
+        "docsect3": visit_docsectX,
         "docsimplesect": visit_docsimplesect,
         "doctitle": visit_doctitle,
         "docformula": visit_docformula,