Add information on new type of launch button for interactive code

[vlipiainen]

Description

What?

Document the changes made in pull request #142 of aplus-rst-tools, and also add mention of the different kernels available at the moment. Note: merging this doesn't make sense if the other pull request hasn't been merged yet.

Testing

What type of test did you run?

• [x] Manual testing.

Built the course locally, but I didn't test the new type of button yet.

Did you test the changes in

• [ ] Chrome
• [x] Firefox
• [ ] This pull request cannot be tested in the browser.

Think of what is affected by these changes and could become broken

Is it Done?

• [ ] Reviewer has finished the code review
• [ ] After the review, the developer has made changes accordingly
• [ ] Customer/Teacher has accepted the implementation of the feature

Clean up your git commit history before submitting the pull request!

[markkuriekkinen]

Link to the related pull request in a-plus-rst-tools: https://github.com/apluslms/a-plus-rst-tools/pull/142