add an empty line between categories and directives

For consistency with the rest of the doc

Author: rbakbashev

src/attributes.rst

@@ -1704,6 +1704,7 @@ Attribute ``collapse_debuginfo``
 
    CollapseDebuginfoContent ::=
        $$collapse_debuginfo$$  $$($$ CollapseDebuginfoKind $$)$$
+
    CollapseDebuginfoKind ::=
        $$no$$
      | $$external$$

src/lexical-elements.rst

@@ -612,6 +612,7 @@ Byte String Literals
 .. rubric:: Syntax
 
 .. syntax::
+
    ByteStringLiteral ::=
        RawByteStringLiteral
      | SimpleByteStringLiteral
@@ -712,6 +713,7 @@ C String Literals
 .. rubric:: Syntax
 
 .. syntax::
+
    CStringLiteral ::=
        RawCStringLiteral
      | SimpleCStringLiteral