Declare external build dependencies using PEP 725

[garth-wells]

Closes #986.

PEP 735 dependency groups were implemented in #1060. Add the remaining external dependency declarations using the draft PEP 725 format and PEP 804 identifiers: CMake and a C++ compiler for both build layouts, BLAS/LAPACK for the combined build, and the installed Basix C++ library for the Python-only build. CMake continues to enforce C++20 support.

Document the build/host distinction and correct the claim that pip fetches all C++ dependencies. Both external-dependency PEPs remain drafts; this makes requirements available in source metadata without promising automatic system-package installation. The Basix C++ GitHub identifier is valid but is not yet in the prototype central registry, so ecosystem mappings may need to be supplied separately.

Validation:

• Parsed both tables with the PEP 725 reference implementation; checked registered identifiers against the prototype PEP 804 registry and mappings for Ubuntu 24.04, Homebrew and conda-forge.
• Built the combined source distribution and wheel, and the Python-only wheel, in Developer mode; verified both tables survive in the combined source distribution.
• Ran quadrature and Lagrange tests against each newly built wheel.
• Ruff check/format, Gersemi check and git diff whitespace checks.

[jhale]

Ask LLM to cut this back, not needed.