docs: Use StackBlitz for S2, and remove CodeSandbox

[devongovett]

CodeSandbox currently has an issue where if you are not logged in you get an unhelpful error message, but it was the only option for S2 with style macros. https://github.com/devongovett/unplugin-parcel-macros/pull/18 adds a web assembly version of the plugin which works in StackBlitz. This PR adds StackBlitz for S2 and removes CodeSandbox for now. If they address the error, we can add it back in the future.