The html renderer section level used for title numbering is incremented
when entering a new section so that subsections get correct heading
elements. Match the increment with a decrement after having rendered the
section contents, so that multiple consecutive sections would stay
consistent.

Add a test to verify this behaviour.

All content under a raw directive is rendered as-is if the output format
matches the writer. Otherwise none of it is shown.

Text inside an inline or block literal that looks like a substitution
reference must still stay unmodified.