Improve and fix example for <octave-shift>

[lemzwerg]

Here's the current example for <octave-shift>

IMHO, it has two major problems.

• It is incomplete. In particular, it doesn't show where the ending <octave-shift> element should be positioned – probably one of the reasons that lead to the current situation as discussed in `<octave-shift>` issues #552.

• It uses <offset>, which IMHO is bad. The starting position of the '8va' symbol relative to the associated note head should be fixed and not depend on the layout. In other words, <offset> should be replaced with a relative-x attribute.

  

  Of course, using <offset> per se is not wrong (and it probably makes sense in some situations if applied to the end of an ottava so that it stops right before a bar line), but I think it would be best if the MusicXML examples also show good practice. BTW, the value -4 of <offset> in the example is meaningless if the value of <divisions> isn't also given or mentioned.

[infojunkie]

I wrote a scraper for the MusicXML examples at https://github.com/infojunkie/musicxml-midi/blob/main/src/js/musicxml-examples.js

I had to create a MusicXML wrapping to convert the examples to actual MusicXML files. While the generated files are all technically passing validation against musicxml.xsd (except for a few that I flagged at #496) I did not verify (and highly doubt) that they make sense semantically.

I do think the examples would be more useful to the community if they included the full MusicXML structures, instead of fragments.