Structured way for documentation begin/ends instead of markers

The html output of QDoc is used by QtCreator to provide help on hovering tokens. Also, QML LSP is currently implementing lsp-documentation-hints relying on qdoc's html output.

Unfortunately, there is no a structural way to extract documentation info from html files. QtCreator extracts documentation relying on some markers that }}{{QDoc writes to denote the beginning and the end of an elements. For example,

// qtobject.html
<!-- $$$QtObject-brief -->
<p>A basic QML type. <a href="#details">More...</a></p>
<!-- @@@QtObject --> 

To extract "Basic QML type.", we first find the start mark <!-- $$$QtObject-brief --> and do the processing until the end mark. But this wouldn't work in case there is a code snippet before using this snippet.

// snippet
Button {
    text: "Hit the button to get 🤑<!-- $$$QtObject-brief -->🤑"
}?

Although this could be a rare case, we still need a structural way to obtain documentations for elements from qdoc-htmls without relying on markers.