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/content.scss

@@ -3,7 +3,7 @@
 //
 // Styles for rendered markdown in the .main-content container
 //
-// stylelint-disable selector-no-type, max-nesting-depth, selector-max-compound-selectors, selector-max-type
+// stylelint-disable selector-no-type, max-nesting-depth, selector-max-compound-selectors, selector-max-type, selector-max-specificity, selector-max-id
 
 .main-content {
   line-height: $content-line-height;
@@ -204,19 +204,17 @@
   h3,
   h4,
   h5,
-  h6 {
+  h6,
+  #toctitle {
     position: relative;
     margin-top: 1.5em;
     margin-bottom: 0.25em;
 
-    &:first-child {
-      margin-top: $sp-2;
-    }
-
     + table,
     + .table-wrapper,
     + .code-example,
-    + .highlighter-rouge {
+    + .highlighter-rouge,
+    + .sectionbody .listingblock {
       margin-top: 1em;
     }
 
@@ -224,4 +222,18 @@
       margin-top: 0;
     }
   }
+
+  > h1:first-child,
+  > h2:first-child,
+  > h3:first-child,
+  > h4:first-child,
+  > h5:first-child,
+  > h6:first-child,
+  > .sect1:first-child > h2,
+  > .sect2:first-child > h3,
+  > .sect3:first-child > h4,
+  > .sect4:first-child > h5,
+  > .sect5:first-child > h6 {
+    margin-top: $sp-2;
+  }
 }