Update field definitions page

[peterdesmet]

The page https://inbo.github.io/etn/articles/etn_fields.html needs a number of updates:

• It should return the tables as returned by the functions
• It should link the field names with the field names + definitions stored in app.field_metadata. The order should be defined by the function, not by a column in app.field_metadata
• The field names in app.field_metadata should be updated to what they are in the functions.