revise xref-group in tex-math

[pkra]

Since we didn't have a use case yet, xref-group was not considered inside tex-math.

Now that texml always wraps xref in xref-group, we should check if we're happy with that (cf. #455).

[pkra]

It might allow us to move from our custom xhref macro to MathJax's regular href macro.

[pkra]

It might allow us to move from our custom xhref macro to MathJax's regular href macro.

I wish my past self would let me know what it had in mind here. Maybe use xref-group to create a macro for a wrapping element (maybe a box?) with a data-attribute so we can separate the link from the data?