1. Fix incorrect flow for Bold and Strong tags
   KDoc currently renders Strong as MarkdownElementTypes.STRONG->B(DocTag)->TextStyle.Strong, but should be MarkdownElementTypes.STRONG->Strong(DocTag)->TextStyle.Strong
   Html tags <b> and <strong> from Javadoc are rendered as:
   <b>->B->TextStyle.Strong, should be <b>->B->TextStyle.Bold
<strong>->Strong->nowhere, should be <strong>->Strong->TextStyle.Strong
2. <em> html tag from Javadoc is converted in HtmlToDocTagConverter.kt to Em on Documentables model, but is not converted further to Pages. New TextStyle.Emphasis is needed to be created.
   Additionally, MarkdownElementTypes.EMPH-> I(children, params) should now be converted to Em DocTag and further into TextStyle.Emphasis.
3. Add support for remaining header html tags <h4-6>.

Related: #2262