gh-134160: "First extension module" tutorial improvements (GH-144183)

- Pass -v to pip, so compiler output is visible
- Move the call ``spam.system(3)`` up so that error handling is tested
  right after it's added
- Use `PyUnicode_AsUTF8AndSize` as `PyUnicode_AsUTF8` is not in the
  Limited API.
- Add a footnote about embedded NULs.