Investigate support for more javadoc html tags

[IgnatBeresnev]

At the moment, there's inconsistency and confusion when it comes to javadoc html tags support

There are basically 3 kinds of tags atm:

• Those we support natively (we fully parse & custom render it)
• Those we don't parse, but can render. For instance, tag <br> is not parsed and is therefore lost, but we have support for it internally (same implementation as for MarkdownTokenTypes.HARD_LINE_BREAK). Same for strikethrough. These should be simple fixes
• Those we don't support at all. They and their children get lost and an empty text tag is returned.

Proposal:

1. Investigate which tags already have render support (like br and strike) or where very little work is needed
2. Create a list of tags that don't have any support, try to sort it in order of importance/frequency of use. Some of these should probably be implemented as separate issues.
3. Investigate whether it's possible to render unsupported html tags the same way it's rendered for kotlin (through passing raw html text with a certain content type).

[IgnatBeresnev]

When/If adding support for <div> tag, write a test for

<dl>
    <div>
        <dt>Term1</dt>
        <dd>Desc2</dd>
    </div>
    <div>
        <dt>Term2</dt>
        <dd>Desc2</dd>
    </div>
</dl> 

At the moment it's impossible to do so because div is not supported and all its children are lost

[IgnatBeresnev]

Just found out there's more missing javadoc tags in a similar (if not duplicate) issue planned for stable: #1618

[asfalcone]

Please add support for header elements <h1> .. <h6>

[daymxn]

+1 to adding support for <br>