Add rubric type="marginal" to guidelines and schema

[wlpotter]

@davidamichelson we at one point discussed using rubric[@type="marginal"] for cases where Wright tells us that there is a marginal rubric.

Currently haven't used in data, but we have several examples of child tei:locus with text node "Marginal", several child tei:note indicating there is marginal content, and the presence of Wright's "marg." abbreviation in rubrics.

Do we want to add to documentation and schema the use of this typology? Or do we want to handle such cases a different way?

[wlpotter]

https://bl.syriac.uk/ms/622 is an example where several marginal rubrics were noted in tei:note, instead of in tei:rubric