Revise chapter 13

[musicEnfanthen]

This PR proposes changes for the revision of chapter 13: Linking data, of the MEI guidelines.

In detail, it contains the following changes:

• It restructures the chapter with two main sections Linking from MEI & Linking into MEI
• It moves the former Pointer & References section to the Linking from MEI part.
• It updates the available attributes for <ptr>and <ref> and the explanations.
• It removes all outdated references or declares them as deprecated.
• It adds placeholders for the Linking into MEI section. @musicog and me will have another look at this section next week.

This PR does not go on master, but on a dedicated branch as it is still work in progress. After replacing the stubs in the Linking into MEI section with real content, we will provide a new PR for master.

[musicEnfanthen]

@musicog Could you have a look at these changes?

[bwbohl]

@musicEnfanthen do I understand correctly that you just want this merged to this repo for convenience?