docs: 📝 Add MailerLite guide

apps/docs/docs/guides/_category_.json

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+{
+  "label": "Guides / Tutorials",
+  "position": 5
+}

apps/docs/docs/guides/mailer-lite.md

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+# Add a subscriber to MailerLite
+
+1. Add a step that collects the email and set it into a variable
+2. Add a Webhook step
+
+<img src="/img/guides/mailer-lite/1.png" width="800" alt="Iframe preview"/>
+
+3. Configure the Webhook step with the following information:
+
+  <img src="/img/guides/mailer-lite/2.png" width="400" alt="Iframe preview"/>
+  
+  For more info on what fields you can add: https://developers.mailerlite.com/reference/create-a-subscriber
+
+4. Replace "YOUR_TOKEN" with your API token. It can be found here: https://app.mailerlite.com/integrations/api/
+5. Whenever the user enters his email it should add it to your subscribers' list on MailerLite