User guide: Overview of concepts

[Crocstar]

Suggest getting rid of this section and explaining each term the first time we use it (broadly).

[marqh]

this is documentation of the registry implementation

these details in the registry docs, we can provide a link to those as required

this is software documentation, not service documentation; at most we should provide a link to
https://github.com/UKGovLD/registry-core/wiki

[marqh]

explaining each term the first time we use it (broadly).

i agree, perhaps a link in a footnote registry-core