The sections such as methods and decorator options are already headed
by a H3 heading so we need to map the H3 headings in the API doc source
down to H4 headings.
This commit includes general heading mapping functionality accessible via
the `marked` Nunjucks filter.
PR Close#24000
The autolinking is now done on the `renderedContent` which means it also
captures and autolinks headings that were generated outside of markdown.
PR Close#16336
Pete Bacon Darwin