devel/developer-setup: Make nodejs and yarn installation mandatory

swh-search requires yarn to be installed or its installation fails
so make the nodejs and yarn installation steps mandatory.

docs/devel/developer-setup.rst

@@ -53,18 +53,16 @@ don't need them started globally (this will save you some RAM):
 
   sudo systemctl disable --now cassandra postgresql
 
-If you intend to hack on the frontend part of |swh| Web Applications, you will also
-need to have ``nodejs >= 18`` in your development environment. If the version in your
-Debian-based distribution is lower, you can install node 18 using these commands:
+You must also have ``nodejs >= 18`` in your development environment.
+You can install node 18 using these commands:
 
 .. code-block:: console
 
   curl -fsSL https://deb.nodesource.com/setup_18.x | sudo bash -
   sudo apt install -y nodejs
 
-Also related to Web Applications development, |swh| uses the ``yarn`` package manager
-to retrieve frontend dependencies and development tools. It is recommended to install its
-latest classic version using these commands:
+|swh| uses the ``yarn`` package manager to retrieve frontend dependencies and development tools.
+You must install its latest classic version using this command:
 
 .. code-block:: console