Minor style fixes for jekyll-asciidoc (#829)

I have a site whose content is written in AsciiDoc, using the [jekyll-asciidoc][] plugin.

Just the Docs works great, but there are just two minor styling glitches I've noticed:

The first is that Just the Docs' CSS doesn't understand the code block markup jekyll-asciidoc produces.  It's not too different though, so it's very easily fixed.

The second is that jekyll-asciidoc generates `div.sect(𝑛 − 1)` elements around headings of type `h𝑛`, that enclose all the heading and all the content after it until the next heading of greater or equal rank.

This means that headings are _always_ first children in AsciiDoc output, which meant the wrong margins were applied to most headings. To fix this, we need to only reduce the margin of first-child headings nested directly below the .main-content element, and headings nested directly below AsciiDoc `.sect𝑛` elements that are themselves first children.

With these two small changes, my site looks perfect, and the styles look exactly the same as on Just the Docs' own documentation.

[jekyll-asciidoc]: https://github.com/asciidoctor/jekyll-asciidoc