Fix margings of AsciiDoc headers

What does this MR do?

Fixes styling of AsciiDoc headers on all pages that allows to use AsciiDoc markup.

Are there points in the code the reviewer needs to double check?

This patch introduces some new CSS classes, one for each supported markup: adoc, textile, rdoc, org, creole, mediawiki, rst, plain. There’s already a CSS class md for Markdown, so I’ve followed this pattern. However, you might prefer to prefix these classes, e.g. markup-adoc…?

Why was this MR needed?

Headers on AsciiDoc pages lacks top margin, so the text is very tight and hard to read. This is an unwanted side effect of the following style rule for Markdown:

.wiki *:first-child {
    margin-top: 0;
 }

Screenshots (if relevant)

The current state

After applying this MR

/cc @mojavelinux