Docs: Add dokiwiki upgrade guide

docs/guides/update-dokuwiki.md

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+---
+title: Updating Dokuwiki
+---
+
+!!! success "Goal"
+
+    How to update our wiki (https://wiki.hamburg.ccc.de) to the latest version
+
+## 1. Create a Snaphot
+
+Go to the Chaosknoten Proxmox UI and create a snapshot of the `wiki` VM: https://chaosknoten.hamburg.ccc.de:8006/#v1:0:=qemu%2F127:4:::::29::
+
+## 2. Update Plugins
+
+- Go to the wiki's extension manager: https://wiki.hamburg.ccc.de/start?do=admin&page=extension or go to Admin > Extension Manager
+- Update the "DokuWiki Upgrade Plugin" (if it has updates)
+- Update other plugins while we're here already
+
+## 3. Update Dokuwiki
+
+- Go to the Wiki Upgrade: https://wiki.hamburg.ccc.de/start?do=admin&page=upgrade or go to Admin > Wiki Upgrade
+- Click through the steps. Note: Disabling any plugins before the upgrade was not necessary in the past.
+
+## Closing Up
+
+Log out and back in again to verify that the Keycloak connection still works.
+Maybe browse some pages as check if anything is obviously broken.