add a warning about outdated docs

Author: TheElectronWill

src/start/config.md

@@ -3,6 +3,13 @@
 Alumet and its plugins can be configured by modifying a [TOML](https://toml.io/en/) file.
 If you don't know TOML, it's a configuration format that aims to be "a config file for humans", easy to read and to modify.
 
+<div class="warning">
+
+For now, this User Book documents the last release of Alumet.
+To get the documentation of a previous release, go to the GitHub repository, [navigate to the desired release](https://github.com/alumet-dev/alumet/releases), and open the README.md of a plugin.
+
+</div>
+
 ## Where is the configuration file?
 
 ### Packaged Alumet agent