This is a big documentation update. I have:

• Rewritten large parts of the build instructions
  • Dead links have been updated.
  • Version numbers and ranges bumped.
  • Instructions simplified (scattered bits of wisdom consolidated)
  • LLVM build options set to match the pip wheels.
  • Note that WebAssembly is required now.
• Moved the README_*.md files to doc/ and added links to them to the base README.md
• Split the Hexagon documentation into its own doc/Hexagon.md.
• Split README_cmake.md into three more specialized documents (and updated the documentation,
  • Documents CMAKE_OSX_ARCHITECTURES support, c.f. Support CMAKE_OSX_ARCHITECTURES #8390
• Updated the documentation in README_python.md ~> doc/Python.md.
  • Documents how to build and install a wheel locally, c.f. Pip packaging at last! #8405
  • Documents how to install from PyPI (and Test PyPI)
• Updated the CMake install rules to include this documentation

The best way to review this PR will be to actually read the documentation, which is navigable starting from here:

• https://github.com/halide/Halide/blob/alexreinking/documentation/README.md

Additional files with substantial changes are here:

• https://github.com/halide/Halide/blob/alexreinking/documentation/doc/BuildingHalideWithCMake.md
• https://github.com/halide/Halide/blob/alexreinking/documentation/doc/CodeStyleCMake.md
• https://github.com/halide/Halide/blob/alexreinking/documentation/doc/HalideCMakePackage.md
• https://github.com/halide/Halide/blob/alexreinking/documentation/doc/Python.md