Bring over PEPs 517, 518, and 660 to the specs section

As of right now various specifications reference those PEPs but there is no complete view of what they contain (e.g. the sdist spec references PEP 518 for what pyproject.toml although technically it should also reference PEP 517 for being to actually build an sdist from a source tree). We should probably have a single specification that covers how to build sdists and wheels from a source tree which the sdist and wheel format specs can reference on how to end up with those binary artifacts.

まず、PEPs 517、518、660 と、specs セクションにある既存の sdist 仕様および wheel 形式仕様を確認します。ソースツリーから sdists と wheels をビルドするための統一仕様の範囲を決定し、既存の仕様がそれを参照するように関連する参照を更新します。