Update Chapter 05 (dev) - Repertoire - Mensural Notation

[martha-thomae]

This PR updates the Mensural Notation chapter so that it is consistent with the changes in the schema. The main changes contained in this PR are:

• Removed the @num and @numbase attributes from the examples containing @dur.quality.
• Edit the sections 5.2 Mensurations and 5.3 Proportions to reflect the schema (which was changed by this revert commit https://github.com/music-encoding/music-encoding/commit/2a58dfbfd13c19dc2be0000349f70d3111f8b0ec). Changed the text of these two sections to something close to the text in version 4.0.1.
• Also changed all the examples encoding mensuration values (again, due to the previously mentioned revert commit). Mensuration was moved from <mensur> into <staffDef> once again.

[martha-thomae]

This branch has a very-confusing name, sorry. I named it after the first change I did. The first commit is about dur.quality, although the rest of the commits are for mensuration- and proportion-related changes.

[bwbohl]

@martha-thomae maybe someone on the mesural-ig can double check these changes with the current development version of the schema?