Improve Markdown component

[mtsknn]

The component is called Markdown, but it takes HTML instead of Markdown, so it should be called e.g. Html.

Also consider changing the unnecessary wrapper <div>/<span> tag to e.g. <my-html> and removing the custom <my-html> tags in eleventy/preact.js, similarly to how <style id="__twind"></style> is replaced with proper Twind-generated styles. Like this:

• https://github.com/mtsknn/mtsknn.fi/blob/e451c26/eleventy/preact.js#L18-L20
• https://github.com/mtsknn/mtsknn.fi/blob/e451c26/components/Markdown.js#L6-L11