Make example environment print code (final feature for 1.2)

Make the `example` environment print code next to its output. With highlighting, if possible.

- [x] Example environment prints (highlighted) code next to example *(in 37ee623aaa)*
- [x] Remove the package indicator for examples *(in 9403e56b787a65cba05729a0faad9bdd4bc47499)*
- [x] Potentially remove the line numbers for examples *(in 9403e56b787a65cba05729a0faad9bdd4bc47499)*
- [x] Possibly vertically center them wrt each other *(in f72a139067a6194c86049a9d936becf761f31e9f)*
- [x] Style it better, that black rule is not nice *(in 9403e56b787a65cba05729a0faad9bdd4bc47499 and f72a139067a6194c86049a9d936becf761f31e9f)*
- [x] Force example code into minipage, typeset side-by-side with the result *(in 1ea5d67203e87bf1e5adaebe5630657d3bb26fc4, with coffins)*
- [x] Document changes in documentation *and* implementation — this includes the usual "no whitespace before `\end{verbatim}`" caveat but for `example` instead. *(in f72a139067a6194c86049a9d936becf761f31e9f and 5285fd6bd0827f65a63ad044095d9ee1dd6fea5e)*
- [x] Clean up the mess left in `\skdoc@verbatim` (*some of it in c78c0a6, some in f72a139067a6194c86049a9d936becf761f31e9f — fine after 6686f36db0d0318ef7858c11f56139c199f239bf*)