Improve performance by rendering markdown partially

[CNSeniorious000]

Use MarkdownIt.parse to implement this.

[linear[bot]]

PROM-31 Improve performance by rendering markdown partially

[CNSeniorious000]

Or we can try using another markdown parser like marked.

This is because MarkdownIt.parse's result is somehow not ready to render: it produces a list of tokens, but we want a nested tree. Although transforming that should be easy enough, it may hurt performance.