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

_sass/code.scss

@@ -38,6 +38,9 @@ a:visited code {
 //   > td.gutter.gl > pre.lineno
 //   | td.code > pre
 //
+// ----...---- (AsciiDoc)
+//   div.listingblock > div.content > pre.rouge.highlight
+//
 // fix_linenos removes the outermost pre when it encloses table.rouge-table
 //
 // See docs/index-test.md for some tests.
@@ -46,7 +49,8 @@ a:visited code {
 // Kramdown line_numbers = true: fences have a wider gutter than with Liquid?
 
 // ```[LANG]...```
-div.highlighter-rouge {
+div.highlighter-rouge,
+div.listingblock {
   padding: $sp-3;
   margin-top: 0;
   margin-bottom: $sp-3;
@@ -123,6 +127,7 @@ figure.highlight {
   border-radius: $border-radius;
 
   + .highlighter-rouge,
+  + .sectionbody .listingblock,
   + figure.highlight {
     position: relative;
     margin-top: -$sp-4;