docs: Fix multiple issues with the clan installation guide

docs/site/getting-started/check.md

@@ -0,0 +1,27 @@
+### Generate Facts and Vars
+
+Typically, this step is handled automatically when a machine is deployed. However, to enable the use of `nix flake check` with your configuration, it must be completed manually beforehand.
+
+Currently, generating all the necessary facts requires two separate commands. This is due to the coexistence of two parallel secret management solutions: the older, stable version (`clan secrets` and `clan facts`) and the newer, experimental version (`clan vars`).
+
+To generate both facts and vars, execute the following commands:
+
+```sh
+clan facts generate && clan vars generate
+```
+
+
+### Check Configuration
+
+Validate your configuration by running:
+
+```bash
+nix flake check
+```
+
+This command helps ensure that your system configuration is correct and free from errors.
+
+!!! Tip
+
+    You can integrate this step into your [Continuous Integration](https://en.wikipedia.org/wiki/Continuous_integration) workflow to ensure that only valid Nix configurations are merged into your codebase.
+