#168 Index of TEI elements: enhancements

[BODARD Gabriel]

A few useful enhancements to the Index of TEI elements (https://epidoc.stoa.org/gl/latest/app-elements.html):

• filter index so only English pages linked from English index, Portuguese from Portuguese index, ecc.
• add short gloss of element names (from ODD)
• add link to EpiDoc schema page for elements

I suspect this is simple enough XSLT that I'm happy to look at in the first instanc.e. (Language filter is related to ticket bugs:178, but needs more robust solution.)