update doc with deb822 repository format (#249)

Author: David Coutadeur

docs/installation.rst

@@ -47,15 +47,25 @@ Import the PGP key:
     apt install curl gpg
     curl https://ltb-project.org/documentation/_static/ltb-project-debian-keyring.gpg | gpg --dearmor > /usr/share/keyrings/ltb-project-debian-keyring.gpg
 
-Configure the repository:
+Configure the apt repository:
 
 .. prompt:: bash #
 
-    vi /etc/apt/sources.list.d/ltb-project.list
+    vi /etc/apt/sources.list.d/ltb-project.sources
 
 .. code-block:: ini
 
-    deb [arch=amd64 signed-by=/usr/share/keyrings/ltb-project-debian-keyring.gpg] https://ltb-project.org/debian/stable stable main
+    Types: deb
+    URIs: https://ltb-project.org/debian/stable
+    Suites: stable
+    Components: main
+    Signed-By: /usr/share/keyrings/ltb-project-debian-keyring.gpg
+
+.. note::
+
+    You can also use the old-style source.list format. Edit ``ltb-project.list`` and add::
+
+        deb [arch=amd64 signed-by=/usr/share/keyrings/ltb-project-debian-keyring.gpg] https://ltb-project.org/debian/stable stable main
 
 Then update: