docs/concept/unification: CUE's core operation

[jpluscplusm]

A new 3-part concept guide explaining unification:

Foundation

• syntax: explicit vs implicit
• results of unifying basic types versus structs vs lists
• order irrelevant

  Detail

• anything else implied or stated in https://alpha.cuelang.org/docs/reference/spec/#unification
• non-concrete fields can be unified
• building up a struct from multiple locations

  Advanced

• failure to unify is only important if a concrete reference to the failure is used
• definitions are closed at time of reference, but unified from multiple locations at time of creation