Mention world location tech debt
Mention world location tech debt

This extends the section on problems with world taxons to mention
the tech debt card, which has some more information about the conflict
between the two. Ideally we should rewrite this manual to be about both
concepts, and this is just an interim measure.

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