Doc: Multi-Dim and Python Executable

Documents how to overwrite the SPACEDIM default of AMReX from "3"
to building 1, 2 and 3 dimensional
versions at the same time.
Example: `-DAMReX_SPACEDIM="1;2;3"`

Documents how to overwrite the automatically found Python executable,
which is useful if one is picked that is undesired. Typically happens
outside of virtual environments and on macOS.
Example: `-DPython_EXECUTABLE=$(which python3)`

README.md

@@ -135,7 +135,7 @@ python3 -m pip install -v --force-reinstall --no-deps .
 
 If you are iterating on builds, it will faster to rely on ``ccache`` and to let CMake call the ``pip`` install logic:
 ```bash
-cmake -S . -B build
+cmake -S . -B build -DAMReX_SPACEDIM="1;2;3"
 cmake --build build --target pip_install -j 8
 ```
 
@@ -168,7 +168,7 @@ If you are using the pip-driven install, selected [AMReX CMake options](https://
 | `AMREX_GPU_BACKEND`          | **NONE**/SYCL/CUDA/HIP                     | On-node, accelerated GPU backend                             |
 | `AMREX_MPI`                  | ON/**OFF**                                 | Enable MPI                                                   |
 | `AMREX_PRECISION`            | SINGLE/**DOUBLE**                          | Precision of AMReX Real type                                 |
-| `AMREX_SPACEDIM`             | 1/2/**3**                                  | Dimension of AMReX                                           |
+| `AMREX_SPACEDIM`             | **3**, use `"1;2;3"` for all               | Dimension(s) of AMReX as a ``;``-separated list              |
 | `AMREX_BUILD_SHARED_LIBS`    | ON/**OFF**                                 | Build the core AMReX library as shared library               |
 | `AMREX_SRC`                  | *None*                                     | Absolute path to AMReX source directory (preferred if set)   |
 | `AMREX_REPO`                 | `https://github.com/AMReX-Codes/amrex.git` | Repository URI to pull and build AMReX from                  |
@@ -179,6 +179,7 @@ If you are using the pip-driven install, selected [AMReX CMake options](https://
 | `PYAMREX_LIBDIR`             | *None*                                     | If set, search for pre-built a pyAMReX library               |
 | `PYAMREX_IPO`                | **ON**/OFF                                 | Compile with interprocedural/link optimization (IPO/LTO)     |
 | `PYINSTALLOPTIONS`           | *None*                                     | Additional options for ``pip install``, e.g., ``-v --user``  |
+| `Python_EXECUTABLE`          | (newest found)                             | Path to Python executable                                    |
 
 Furthermore, pyAMReX adds a few selected CMake build options: