Heading introduces a new section. Headings go to the table of contents.
Heading ::=
(Heading1
| Heading2
| Heading3
| Heading4) NL
Heading1 ::= "=" LWS FormattedLine
Heading2 ::= "==" LWS FormattedLine
Heading3 ::= "===" LWS FormattedLine
Heading4 ::= "====" LWS FormattedLine