Bug 1750874: Support launching Mach with `py` instead of `python` r=glandium

When installing from Python.org, the `python` command isn't available by
default (in favour of `py`). Support using `py` instead.

There were issues running `hg` when `python` isn't added to the `PATH`,
so the docs are being updated accordingly.
Of course, it's still worth supporting `py`-instead-of-`python`
workflows for `git` users who won't be impacted by that bug.

Differential Revision: https://phabricator.services.mozilla.com/D136286

mach.cmd

 @ECHO OFF
 SET topsrcdir=%~dp0
-python %topsrcdir%mach %*
+
+WHERE /q py
+IF %ERRORLEVEL% EQU 0 (
+  py %topsrcdir%mach %*
+) ELSE (
+  python %topsrcdir%mach %*
+)

mach.ps1

 $mypath = $MyInvocation.MyCommand.Path
-$machpath = ($mypath -replace '\\', '/').substring(0, $mypath.length - 4)
+$machpath = $mypath.substring(0, $mypath.length - 4)
+
+if (Get-Command py) {
+  $python_executable = "py"
+} else {
+  $python_executable = "python"
+}
 
 if (-not (test-path env:MACH_PS1_USE_MOZILLABUILD)) {
-  python $machpath $args
+  &$python_executable $machpath $args
   exit $lastexitcode
 }
 
@@ -11,6 +17,8 @@ if (-not (test-path env:MOZILLABUILD)) {
   exit 1
 }
 
+$machpath = ($machpath -replace '\\', '/')
+
 if ($machpath.contains(' ')) {
   echo @'
 The repository path contains whitespace which currently isn't supported in mach.ps1.

python/mach/docs/windows-usage-outside-mozillabuild.rst

@@ -55,7 +55,8 @@ Download Python from the `the official website <https://www.python.org/downloads
     version is 3.10.1, so a safe version to install would be the most recent 3.9 release.
 
 You'll want to download the "Windows installer (64-bit)" associated with the release you've chosen.
-During installation, ensure that you check the "Add Python 3.x to PATH" option.
+During installation, ensure that you check the "Add Python 3.x to PATH" option, otherwise you might
+`encounter issues running Mercurial <https://bz.mercurial-scm.org/show_bug.cgi?id=6635>`__.
 
 .. note::