Add support for document type "mise-en-pratique"

[ronaldtse]

SI Brochure Appendix 2 contains a number of independent documents, seen here:
https://www.bipm.org/en/publications/mises-en-pratique/

For each SI Unit, there is 1 document published as a Mise en pratique (MEP). Each MEP may be accompanied by additional guidance documents, but we consider those separately.

The sources for MEPs are provided here:
https://github.com/metanorma/bipm-si-brochure/tree/master/sources with the prefix mep-.

We need to unify a document layout for all MEPs and probably for all other "non-brochure" document types.

[opoudjis]

Not seeing any great idiosyncrasies in the documents, the versions online are straightforward Word documents with autoformatting.

[opoudjis]

Review of #5 is in effect review of these MEPs, so closing this ticket.