LibManual: Refactor SectionNode in preparation for subsections

- Calculate the full name on demand
- Make section and name protected
- Reorder some members logically
- Change the name getter to be fallible, as some implementors need to
  allocate

Userland/Libraries/LibManual/SectionNode.cpp

@@ -17,6 +17,11 @@ ErrorOr<String> SectionNode::path() const
     return String::formatted("/usr/share/man/man{}", m_section);
 }
 
+ErrorOr<String> SectionNode::name() const
+{
+    return String::formatted("{}. {}", m_section, m_name);
+}
+
 ErrorOr<void> SectionNode::reify_if_needed() const
 {
     if (m_reified)