Relocate spack_cmd and scripts from installer to root bin (#24651)

Relocate spack_cmd and scripts from installer
to root bin

Refactor documentation, installer, and launcher
to facilitate that change

bin/spack_cmd.bat

@@ -0,0 +1,61 @@
+@ECHO OFF
+setlocal EnableDelayedExpansion
+:: (c) 2021 Lawrence Livermore National Laboratory
+:: To use this file independently of Spack's installer, execute this script in its directory, or add the
+:: associated bin directory to your PATH. Invoke to launch Spack Shell.
+::
+:: source_dir/spack/bin/spack_cmd.bat
+::
+pushd %~dp0..
+set SPACK_ROOT=%CD%
+pushd %CD%\..
+set spackinstdir=%CD%
+popd
+
+
+:: Check if Python is on the PATH
+(for /f "delims=" %%F in ('where python.exe') do (set python_pf_ver=%%F) ) 2> NUL
+
+if not defined python_pf_ver (
+    :: If not, look for Python from the Spack installer
+    :get_builtin
+    (for /f "tokens=*" %%g in ('dir /b /a:d "!spackinstdir!\Python*"') do (
+        set python_ver=%%g)) 2> NUL
+
+    if not defined python_ver (
+        echo Python was not found on your system.
+        echo Please install Python or add Python to your PATH.
+    ) else (
+        set py_path=!spackinstdir!\!python_ver!
+        set py_exe=!py_path!\python.exe
+    )
+    goto :exitpoint
+) else (
+    :: Python is already on the path
+    set py_exe=!python_pf_ver!
+    (for /F "tokens=* USEBACKQ" %%F in (
+        `!py_exe! --version`) do (set "output=%%F")) 2>NUL
+    if not "!output:Microsoft Store=!"=="!output!" goto :get_builtin
+    goto :exitpoint
+)
+:exitpoint
+
+set "PATH=%SPACK_ROOT%\bin\;%PATH%"
+if defined py_path (
+    set "PATH=%py_path%;%PATH%"
+)
+
+if defined py_exe (
+    "%py_exe%" "%SPACK_ROOT%\bin\haspywin.py"
+    "%py_exe%" "%SPACK_ROOT%\bin\spack" external find python >NUL
+)
+
+set "EDITOR=notepad"
+
+DOSKEY spacktivate=spack env activate $*
+
+@echo **********************************************************************
+@echo ** Spack Package Manager
+@echo **********************************************************************
+
+%comspec% /k