Force UTF-8 encoding on title and markdown of imported docs
Force UTF-8 encoding on title and markdown of imported docs

Some docs have different encoding, which breaks Middleman.
This fix takes the same approach as ExternalDoc does here:
https://github.com/alphagov/govuk-developer-docs/blob/34b8e6ebaf85e4c81081dacc722b8f574f9e4699/app/external_doc.rb#L38

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs