Render keyboard keys

[ralf401]

In the RTD docs, the keys can be rendered like that:

It would be nice to have it here as well. I already investigated but could not solve it. One source pointed out to use [[x]], but it didn't work.
For the occasions where I needed it, I used <kbd>x</kbd>. It got preserved in the html, but not rendered as a key. Perhaps there is a style for that missing?