[doc] Show getting-started with modern python venv

Also show `pip install <url-for-tar.gz-of-master-branch-on-gitlab>`.

doc/getting-started.rst

@@ -28,11 +28,15 @@ The recommended way to install Buildout is to use pip within a virtual environme
 
 .. code-block:: console
 
-  virtualenv mybuildout
-  cd mybuildout
-  bin/pip install zc.buildout
+  python3 -m venv myenv
+  source myenv/bin/activate
+  pip install zc.buildout
 
+Or for the code from master branch:
 
+.. code-block:: console
+
+  pip install https://lab.nexedi.com/nexedi/slapos.buildout/-/archive/master/slapos.buildout-master.tar.gz
 
 To use Buildout, you need to provide a Buildout configuration. Here is
 a minimal configuration: