Handbook priorities for 4.0 release

We're getting close! I've gone through the spreadsheet (https://docs.google.com/spreadsheets/d/1SBlekCrN8CAYokinRjVWMOYR4UA4_6Q…) and added an "X" in the "High Priority" column for the pages we're most wanting to see completed before release. We'd love to see more people get involved - but for the record, geetar and dulciepercy are rock stars here (or stars of your preferred musical genre :-)

Also, much about basic notation has not changed since MuseScore 3, so thre are quite a few pages where you can pretty much copy content from the old Handbook to the new. I'd love to keep the new heading structure if possible, though - so copy the text into the new headings. If we get desperate towards release we can possibly relax that just to have something.

But also, it doesn't have to be perfect the day of release. After all, we know MuseScore 4.0 itself will have bugs, and a few missing features to be added back. The Handbook can be similar. Let's focus on making it great rather than just rushing it to completion - but, let's focus on the pages I've marked where I think it will be most valuable to have content soon.

The link to the Handbook itself is https://musescore.org/en/draft-handbook-mu4, and you'll find any further instructions you need for editing there.