[spec] conceptual foundation and glossary entries for variance

Currently the section on variance says:

A longer introduction to these concepts can be found on Wikipedia and in PEP 483; here we just show how to control a type checker’s behavior.

Now that we have a core concepts section in the spec, it would be good for the spec to also derive the rationale for variance from those core concepts, rather than deferring to the (ten-year-old and not updatable) PEP 483.

We should also add glossary entries for variance terminology: invariant, covariant, contravariant, and variance.

仕様の分散に関するセクションと既存のコア概念のセクションから始め、次に PEP 483 とリンクされた Wikipedia の記事にある根拠を比較します。分散のセクションがコア概念からその根拠を導出し、用語集が不変、共変、反変、分散を定義すれば完了です。