playground: fill placeholder with an example djot document

[iceghost]

I think it would be nice if there is a djot document to play around with, readme.dj for example, rather than a blank document.

I suggest we fetch the file from GitHub on page load.

Some alternative:

• Inline it inside the HTML in the make build process.
• Fetch on demand, i.e add some sort of buttons or selects (this could be better to extend to more examples file)

I could try a PR if it is fine.

[jgm]

We used to have some sample text, but I removed it. I don't recall why. I'll think about it.