User guide: installing: suggest 'pip' when manually installing

'setup.py' is deprecated and has issues (see PR #5790). So, switch over
to 'pip' instead. Also, don't list all possible optional plugins, that
easily gets stale, e.g. 'ansible' was missing already. Just give an
example and tell the user where to look for the optional features.

Signed-off-by: Wolfram Sang <[email protected]>

docs/source/guides/user/chapters/installing.rst

@@ -125,19 +125,18 @@ Then to install Avocado from the git repository run::
 
     $ git clone git://github.com/avocado-framework/avocado.git
     $ cd avocado
-    $ python3 setup.py install --user
+    $ pip install . --user
 
-Optionally, to install the plugins run::
+To install an optional plugin run::
 
-    $ python3 setup.py plugin --install=golang --user
-    $ python3 setup.py plugin --install=html --user
-    $ python3 setup.py plugin --install=result_upload --user
-    $ python3 setup.py plugin --install=resultsdb --user
-    $ python3 setup.py plugin --install=robot --user
-    $ python3 setup.py plugin --install=varianter_cit --user
-    $ python3 setup.py plugin --install=varianter_pict --user
-    $ python3 setup.py plugin --install=varianter_yaml_to_mux --user
+    $ pip install optional_plugins/<plugin_name> --user
 
+I.e. for the HTML plugin::
+
+    $ pip install optional_plugins/html --user
+
+Check the directory ``optional_plugins`` for additional features you might be
+interested in.
 
 .. _Virtualization:Tests project in OpenSUSE build service: https://build.opensuse.org/project/show/Virtualization:Tests
 .. _Avocado-VT: https://avocado-vt.readthedocs.io/en/latest/GetStartedGuide.html#installing-avocado-vt