Explain PYTHONPATH setup in build instructions. (#14834)

As requested [here on
Discord](https://discord.com/channels/689900678990135345/706175572920762449/1144535632039915564).

Also
* Suggest using `help(iree.[compiler|runtime])` to show that import was
successful, rather than just `import`, which has no output
* Write out `.env.ps1` for PowerShell usage on Windows

CMakeLists.txt

@@ -979,9 +979,11 @@ if(IREE_BUILD_PYTHON_BINDINGS)
   set(_PYTHONPATH_ENV "PYTHONPATH=$<SHELL_PATH:${CMAKE_CURRENT_BINARY_DIR}/compiler/bindings/python;${CMAKE_CURRENT_BINARY_DIR}/runtime/bindings/python>\n")
   file(GENERATE OUTPUT "${CMAKE_CURRENT_SOURCE_DIR}/.env" CONTENT "${_PYTHONPATH_ENV}")
   file(GENERATE OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/.env" CONTENT "${_PYTHONPATH_ENV}")
-  # Similarly, write out .env.bat for Windows.
+  # Similarly, write out .env.bat and .env.ps1 for Windows.
   set(_PYTHONPATH_ENV_BAT "set PYTHONPATH=$<SHELL_PATH:${CMAKE_CURRENT_BINARY_DIR}/compiler/bindings/python;${CMAKE_CURRENT_BINARY_DIR}/runtime/bindings/python>\n")
   file(GENERATE OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/.env.bat" CONTENT "${_PYTHONPATH_ENV_BAT}")
+  set(_PYTHONPATH_ENV_PS1 "$env:PYTHONPATH = '$<SHELL_PATH:${CMAKE_CURRENT_BINARY_DIR}/compiler/bindings/python;${CMAKE_CURRENT_BINARY_DIR}/runtime/bindings/python>'\n")
+  file(GENERATE OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/.env.ps1" CONTENT "${_PYTHONPATH_ENV_PS1}")
 endif()
 
 if(IREE_BUILD_BINDINGS_TFLITE)

docs/website/docs/building-from-source/getting-started.md

@@ -360,24 +360,33 @@ Extend your `PYTHONPATH` with IREE's `bindings/python` paths and try importing:
 
     ``` shell
     source ../iree-build/.env && export PYTHONPATH
-    python -c "import iree.compiler"
-    python -c "import iree.runtime"
+    # The 'PYTHONPATH' environment variable should now contain
+    #   iree-build/compiler/bindings/python;iree-build/runtime/bindings/python
+
+    python -c "import iree.compiler; help(iree.compiler)"
+    python -c "import iree.runtime; help(iree.runtime)"
     ```
 
 === ":fontawesome-brands-apple: macOS"
 
     ``` shell
     source ../iree-build/.env && export PYTHONPATH
-    python -c "import iree.compiler"
-    python -c "import iree.runtime"
+    # The 'PYTHONPATH' environment variable should now contain
+    #   iree-build/compiler/bindings/python;iree-build/runtime/bindings/python
+
+    python -c "import iree.compiler; help(iree.compiler)"
+    python -c "import iree.runtime; help(iree.runtime)"
     ```
 
 === ":fontawesome-brands-windows: Windows"
 
     ``` powershell
-    ../iree-build/.env.bat
-    python -c "import iree.compiler"
-    python -c "import iree.runtime"
+    ..\iree-build\.env.ps1  # or ..\iree-build\.env.bat
+    # The 'PYTHONPATH' environment variable should now contain
+    #   iree-build/compiler/bindings/python;iree-build/runtime/bindings/python
+
+    python -c "import iree.compiler; help(iree.compiler)"
+    python -c "import iree.runtime; help(iree.runtime)"
     ```
 
 Using IREE's ML framework importers requires a few extra steps: