Documentation: nixpkgs manual: move Python reference to the top of Python chapter (#247117)

* nixpkgs manual, doc Python: move Reference/Optimizations to FAQ

See https://github.com/NixOS/nixpkgs/issues/246234.

* nixpkgs manual, doc Python: move Reference/python-optional-dependencies to FAQ

See https://github.com/NixOS/nixpkgs/issues/246234.

* nixpkgs manual, doc Python: move Reference/Tools to FAQ

See https://github.com/NixOS/nixpkgs/issues/246234.

* nixpkgs manual, doc Python: move Reference/deterministic-builds to FAQ

See https://github.com/NixOS/nixpkgs/issues/246234.

* nixpkgs manual, doc Python: move Reference/automatic-tests to FAQ

See https://github.com/NixOS/nixpkgs/issues/246234.

* nixpkgs manual, doc Python: move Reference to top section

See https://github.com/NixOS/nixpkgs/issues/246234.