Math rendering in Markdown

[tchamelot]

Hello and thank you for this nice tool,

While testing markdown with math support, I noticed that math equations are not rendered properly.
The generated html looks like

<div class="katex">\Delta</div>

instead of

<span class="katex-display"><span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML" display="block"><semantics><mrow><mi mathvariant="normal">Δ</mi></mrow><annotation encoding="application/x-tex">
 \Delta
</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height: 0.6833em;"></span><span class="mord">Δ</span></span></span></span></span>

Which allow to render the $\Delta$ correctly.

On flexmark-java wiki, it is indicated to add the following script at the end of the generated html page to convert math elements when the DOM is loaded..

<script>
    (function () {
      document.addEventListener("DOMContentLoaded", function () {
        var mathElems = document.getElementsByClassName("katex");
        var elems = [];
        for (const i in mathElems) {
            if (mathElems.hasOwnProperty(i)) elems.push(mathElems[i]);
        }

        elems.forEach(elem => {
            katex.render(elem.textContent, elem, { throwOnError: false, displayMode: elem.nodeName !== 'SPAN', });
        });
    });
})();
</script>

I don't exactly understand how to fix the issue. Extending MarkdownExtension.kt and Page.kt should be enough. I'll try to provide a PR this week-end