docs: add n8n docs

docs/content/doc/usage/n8n.md

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+---
+title: "n8n"
+date: 2023-10-24T19:31:35+02:00
+draft: false
+menu:
+    sidebar:
+        parent: "usage"
+---
+
+# Using Vikunja with n8n
+
+Vikunja maintains a [Community node](https://github.com/go-vikunja/n8n-vikunja-nodes) for [n8n](https://n8n.io),
+allowing you to easily integrate Vikunja with all kinds of other tools and services.
+
+{{< table_of_contents >}}
+
+## Installation
+
+To install the node in your n8n installation:
+
+1. In your n8n instance, go to **Settings > Community Nodes**.
+2. Select Install.
+3. Enter `n8n-nodes-vikunja` as the npm Package Name
+4. Agree to the risks of using community nodes: select I understand the risks of installing unverified code from a
+   public source.
+5. Select Install. n8n installs the node, and returns to the Community Nodes list in Settings.
+6. Vikunja actions and triggers are now available in n8n.
+
+[Official n8n docs about the installation](https://docs.n8n.io/integrations/community-nodes/installation/)
+
+## Authentication
+
+To authenticate your automation against Vikunja:
+
+1. In Vikunja, go to **Settings > API Tokens** and create a new token. Use all scopes for the kind of task you want to
+   do. \
+   *Note:* If you want to use the webhook trigger node, the api token should have permissions to create, read and delete
+   webhooks.
+2. Now in n8n, go to **Credentials** and then click on **Add Credential**.
+3. Search for `Vikunja API` and click *Continue*
+4. Enter the API key you created in step 1.
+5. Enter the API URL of your Vikunja instance, with `/api/v1` suffix.
+6. When you now create a Vikunja node, select the created credentials.