[FIX] base, doc: RTLCSS on Windows

RTLCSS is not working on Windows for several reasons:
- the executable is called `rtlcss.cmd`
- rtlcss is not available in the System Path; it is only available in
  the User Path

closes odoo/odoo#27896

doc/setup/install.rst

@@ -395,6 +395,13 @@ Source installation requires manually installing dependencies:
 
         C:\> npm install -g rtlcss
 
+    It is then necessary to edit the System Environment's variable
+    :envvar:`PATH` and add the folder where `rtlcss.cmd` is located. Typically:
+
+    .. code-block:: console
+
+        C:\Users\<user>\AppData\Roaming\npm\
+
 
 Fetch the sources
 -----------------

odoo/addons/base/models/assetsbundle.py

@@ -525,6 +525,8 @@ class AssetsBundle(object):
 
     def run_rtlcss(self, source):
         rtlcss = 'rtlcss'
+        if os.name == 'nt':
+            rtlcss = misc.find_in_path('rtlcss.cmd')
         cmd = [rtlcss, '-']