Add Bootstrap 5 example

[jtojnar]

Just a note to myself (or to someone else if they beat me to it). Bootstrap 5 has been released a while ago so we might want to update the examples in the repo.

I have already created a more complete renderer over at nextras/forms-rendering so just need to strip it down.

[dg]

Any progress?

[jtojnar]

I did not manage to get to it yet.