Markup corrections in MEP files

[anermina]

• Add unitsml where needed
• Correct formulae in accordance with the original files
• Correct text in accordance with the original files
• Replace unnecessary non-ASCII symbols with their ASCII counterparts
• Added Metrologia references for relaton fetching
• Modified incorrect information (DOI and URI) for some references
• Added missing references
• Guide documents updated to match the files available at https://www.bipm.org/en/publications/mises-en-pratique

Several tickets were filed:

• https://github.com/relaton/relaton-data-bipm/issues/23
• https://github.com/relaton/relaton-data-bipm/issues/24
• https://github.com/relaton/relaton-data-bipm/issues/25
• https://github.com/relaton/relaton-bipm/issues/37
• https://github.com/metanorma/bipm-si-brochure/issues/203
• https://github.com/metanorma/bipm-si-brochure/issues/204

[ronaldtse]

Thank you @anermina !