Update docs/configuring-playbook-sliding-sync-proxy.md: add the common section "Troubleshooting"

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/configuring-playbook-sliding-sync-proxy.md

@@ -68,3 +68,7 @@ You **don't need to do anything special** to make use of the Sliding Sync proxy.
 When the Sliding Sync proxy is [installed](#installing), your `/.well-known/matrix/client` file is also updated. A new `org.matrix.msc3575.proxy` section and `url` property are added there and made to point to your Sliding Sync proxy's base URL (e.g. `https://matrix.example.com/sliding-sync`).
 
 This allows clients which support Sliding Sync to detect the Sliding Sync proxy's URL and make use of it.
+
+## Troubleshooting
+
+As with all other services, you can find the logs in [systemd-journald](https://www.freedesktop.org/software/systemd/man/systemd-journald.service.html) by logging in to the server with SSH and running `journalctl -fu matrix-sliding-sync-proxy`.