Main UI docs: Add log viewer (#2481)

Signed-off-by: Florian Hotze <florianh_dev@icloud.com>

mainui/developer/index.md

@@ -30,4 +30,6 @@ The developer tools page is split into two tabs:
   So if you read this from the website, you can also read this from inside the UI.
 - **API Explorer**<br>
   Interactively discover and access the [REST API]({{base}}/configuration/restdocs), which is also used by Main UI to interact with the openHAB server.
+- [**Log Viewer**](log-viewer)<br>
+  View the logs of the openHAB server in a convenient way. Filter, highlight, download and copy log messages.
 <!-- END MAINUI SIDEBAR DOC - DO NOT REMOVE -->

mainui/developer/log-viewer.md

@@ -0,0 +1,19 @@
+---
+layout: documentation
+title: Developer Tools - Log Viewer
+---
+
+# Developer Tools - Log Viewer
+
+<!-- START MAINUI SIDEBAR DOC - DO NOT REMOVE -->
+Main UI provides an integrated log viewer that allows to view the logs of the openHAB server.
+It provides filtering and highlighting capabilities, as well as download and copy options.
+
+As the log viewer is using WebSockets to receive log messages, some restrictions apply:
+
+- Fancy firewall rules or proxies may block the WebSocket connection.
+- When using HTTPS with a self-signed certificate, your browser may block the WebSocket connection.
+  In some browsers, it is possible to bypass this restriction by setting hidden flags, but unfortunately, this is not possible in all browsers.
+  The only real solution is to use a valid certificate from a trusted certificate authority, which however either requires having a publicly accessible server (not recommended) or having your own CA.
+
+<!-- END MAINUI SIDEBAR DOC - DO NOT REMOVE -->