docs: fix HTML errors (#2732)

* duplicate `title` tag; jekyll-seo-tag adds the title by default
* escape `<`
* fix duplicate IDs
* fix typo `Documentaion`
* add missing `alt` attributes in direct-chat.md

docs/_includes/head.html

@@ -3,7 +3,6 @@
     <meta name="viewport" content="width=device-width, initial-scale=1">
     <meta http-equiv="x-ua-compatible" content="ie=edge">
 
-    <title>{{ page.title }} | AdminLTE 3 Documentation</title>
     {% seo %}
     
     <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Source+Sans+Pro:300,400,400i,700">