Updating VM policy

[vv-monsalve]

This PR includes some adjustments and updates to the Vertical Metrics policies.

Vertical Metrics Requirements

• [x] Requirements reordered
• [x] Since the new minimum GF Latin Core glyphset now includes Vietnamese support. Updating the reference glyph to calculate ascenders value taking Abreveacute as a new reference.
• [x] Updating the notes, adding a reference to stacked diacritics design, and also the first example of concrete cases.
• [x] Pointing at Diffenator2 instead of GF Regressions (which web version seems not to be working) and gen-html

Diacritic marks

• [x] Reinforcing the requirement to include a set of marks for capital letters
• [x] Adding details to the stacked diacritics sub-section.

Links to the rendered files for a better review:

• Diacritics
• Vertical Metrics