[docs] Added a tip to explain how to permanently add environment variables (#22838)

Fixes: #20471

Author: lorsanta

site/source/docs/getting_started/downloads.rst

@@ -52,6 +52,8 @@ GitHub and set them as :term:`active <Active Tool/SDK>`:
     # Activate PATH and other environment variables in the current terminal
     source ./emsdk_env.sh
 
+  .. tip:: If you want to avoid executing `source ./emsdk_env.sh` every time you open a new terminal you can follow the intstructions given by the `emsdk activate` command above to add this command to your startup scripts.
+
   .. note:: On Windows, run ``emsdk.bat`` instead of ``./emsdk``, and ``emsdk_env.bat`` instead of ``source ./emsdk_env.sh``.
 
   .. note:: On Windows, if you use the ``activate`` command, the step of ``emsdk_env.bat`` is optional. If you want to know more, see :ref:`activate SDK version <emsdk-set-active-tools>`.