Docs: Add hyperlink to official HTML specifications for data-* prefix

[thadguidry]

In section https://htmx.org/docs/#introduction , we currently mention that

It’s worth mentioning that, if you prefer, you can use the data- prefix when using htmx:
<a data-hx-post="/click">Click Me!</a>

But there is no reference link for what the data- prefix is. We are assuming our audience has a full understanding of all elements (even newer ones) of HTML, and instead we should be good citizens and hyperlink to standards as necessary when we mention them in our docs.

The link we should refer to the data-* attribute is https://html.spec.whatwg.org/multipage/dom.html#attr-data-*