developer-setup: Add instructions to install latest yarn version

yarnpkg debian package does not play well with nodejs installed from
nodesource so add instructions to install its latest version through
its homemade debian package.

docs/developer-setup.rst

@@ -37,7 +37,7 @@ package manager. On Debian/Ubuntu-based distributions::
       fuse3 libfuse3-dev libcmph-dev libleveldb-dev \
       git myrepos \
       postgresql-autodoc graphviz plantuml inkscape \
-      postgresql libpq-dev cassandra yarnpkg
+      postgresql libpq-dev cassandra
 
 .. Note:: Python 3.7 or newer is required
 
@@ -50,7 +50,7 @@ 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 >= 14 in your development environment. If the version in your
+need to have ``nodejs >= 14`` in your development environment. If the version in your
 Debian-based distribution is lower, you can install node 14 using these commands::
 
   sudo wget https://deb.nodesource.com/gpgkey/nodesource.gpg.key -O /etc/apt/trusted.gpg.d/nodesource.asc
@@ -58,6 +58,15 @@ Debian-based distribution is lower, you can install node 14 using these commands
   sudo apt update
   sudo apt install 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::
+
+  sudo wget https://dl.yarnpkg.com/debian/pubkey.gpg -O /etc/apt/trusted.gpg.d/yarn.asc
+  echo "deb https://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list
+  sudo apt update
+  sudo apt install yarn
+
 If you intend to work on |swh| archive search features, Elasticsearch must also be
 present in your development environment. Proceed as follows to install it::