Mention virtualenv before the install command (#597)

Move the note to use virtual environments before the first `pip` commands, since users
may otherwise install the package globally before reading the note.

2026R1 hardening item 39.

Author: greschd

doc/source/intro.rst

@@ -7,6 +7,9 @@ Installation
 PyDPF - Composites supports Ansys version 2023 R1 and later. Make sure you have a licensed copy of Ansys installed. See
 :ref:`Compatibility` to understand which ``ansys-dpf-composites`` version corresponds to which Ansys version.
 
+You should use a `virtual environment <https://docs.python.org/3/library/venv.html>`_,
+because it keeps Python packages isolated from your system Python.
+
 Install the ``ansys-dpf-composites`` package with ``pip``:
 
 .. code::
@@ -27,10 +30,6 @@ Specific versions can be installed by specifying the version in the pip command.
     pip install ansys-dpf-composites==0.3.0
 
 
-You should use a `virtual environment <https://docs.python.org/3/library/venv.html>`_,
-because it keeps Python packages isolated from your system Python.
-
-
 Examples
 ^^^^^^^^