Add embedded playgrounds on handle and handle_path

[ProfessorLogout]

Hey @mholt and team,

as discussed I took a stab at adding tech playground embedded playgrounds as we discussed in chat.

I've added it two slightly different ways:

• handle_path: Embedded into one of the examples
• handle: Added a separate header "Playground" after the examples, building on it to make it more useful as a playground

I did not do any kind of styling so far, the element works mostly OK just with your existing styles. If we decide to do this as a feature in the docs I would like to take some time to polish things with a bit of custom CSS and add proper styling for the output.

headless.min.js is taken from this projects releases. I'm happy to take care of updates and related testing in the future.

Looking forward to any feedback you want to share!