feat(mattermost): add auth and send message to channel action

* feat: mattermost integration

* post review adjustments

packages/docs/pages/apps/mattermost/actions.md

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+---
+favicon: /favicons/mattermost.svg
+items:
+  - name: Send a message to channel
+    desc: Sends a message to a channel you specify.
+---
+
+<script setup>
+  import CustomListing from '../../components/CustomListing.vue'
+</script>
+
+<CustomListing />

packages/docs/pages/apps/mattermost/connection.md

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+# Mattermost
+
+:::info
+This page explains the steps you need to follow to set up the Mattermost
+connection in Automatisch. If any of the steps are outdated, please let us know!
+:::
+
+1. Go to the `<WORKSPACE_NAME>/integrations/oauth2-apps/add` page of your Mattermost server to register a **new OAuth application**.
+   - You can find details about registering new Mattermost oAuth application at https://docs.mattermost.com/integrations/cloud-oauth-2-0-applications.html#register-your-application-in-mattermost.
+2. Fill in the **Display Name** field.
+3. Fill in the **Description** field.
+4. Fill in the **Homepage** field.
+5. Copy **OAuth Redirect URL** from Automatisch to the **Callback URLs** field on Mattermost page.
+6. Click on the **Save** button at the end of the form on Mattermost page.
+7. Copy the **Client ID** value from the following page to the `Client ID` field on Automatisch.
+8. Copy the **Client Secret** value from the same page to the `Client Secret` field on Automatisch.
+9. Click **Done** button on MAttermost page.
+10. Click **Submit** button on Automatisch.
+11. Congrats! Start using your new Mattermost connection within the flows.