docs: create downgrading instruction

docs/downgrade.md

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+# Downgrading to `v20240206`
+
+## systemd
+
+1. Stop the Firefish service
+    ```sh
+    sudo systemctl stop your-firefish-service.service
+    ```
+1. Take a backup
+1. Switch back to the `v20240206` tag
+    ```sh
+    git switch v20240206
+    ```
+1. Rebuild Firefish
+    ```sh
+    pnpm install --frozen-lockfile
+    NODE_ENV='production' NODE_OPTIONS='--max_old_space_size=3072' pnpm run rebuild
+    ```
+1. Start the Firefish service and confirm that Firefish is downgraded
+    ```sh
+    sudo systemctl start your-firefish-service.service
+    ```
+
+## Docker/Podman
+
+1. Stop the container
+    ```sh
+    docker-compose down
+    # or podman-compose down
+    ```
+1. Take a backup
+1. Change the image tag to `registry.firefish.dev/firefish/firefish:v20240206`
+    ```sh
+    vim docker-compose.yml
+    ```
+
+    ```yaml
+    version: "3"
+
+    services:
+      web:
+        image: registry.firefish.dev/firefish/firefish:v20240206  # here
+    ```
+1. Start the container and confirm that Firefish is downgraded
+    ```sh
+    docker-compose up --detach
+    # or podman-compose up --detach
+    ```