Wrap contents of plain HTML files into contentContainer

There are a number of plain HTML documentation files in java.desktop module, for example Modality.html.

When viewed locally, it looks good: it has margins, in particular left margin.

When it's part of Java API documentation, the text looks flushed to the left side: there are no margins.

I compared it to the reference of a class and found out <div class="contentContainer"> is missing. Adding this wrapper inside <main> produces better looking result which is consistent to other documentation files.