Docs website with mkdocs (#100)

* Fix build workflow

* Docs website with mkdocs (#99)

* Fix docs workflow

* Move mkdocs Docker file

* Ignore docs in build workflow

* Update workflows

* Update links

* Move upgrade notes to documentation

Co-authored-by: CrazyMax <crazy-max@users.noreply.github.com>

docs/notif/slack.md

@@ -0,0 +1,28 @@
+# Slack notifications
+
+You can send notifications to your Slack channel using an [incoming webhook URL](https://api.slack.com/messaging/webhooks).
+
+!!! hint
+    Mattermost webhooks are compatible with Slack notification without any special configuration (if Webhooks are enabled).
+
+## Configuration
+
+!!! example "File"
+    ```yaml
+    notif:
+      slack:
+        webhookURL: https://hooks.slack.com/services/ABCD12EFG/HIJK34LMN/01234567890abcdefghij
+    ```
+
+!!! abstract "Environment variables"
+    * `DIUN_NOTIF_SLACK_WEBHOOKURL`
+
+| Name               | Default       | Description   |
+|--------------------|---------------|---------------|
+| `webhookURL`[^1]   |               | Slack [incoming webhook URL](https://api.slack.com/messaging/webhooks) |
+
+## Sample
+
+![](../assets/notif/slack.png)
+
+[^1]: Value required