Added documentation for Permalinks extension [ci skip]

docs/extensions/permalinks.md

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+# Permalinks
+
+Permalinks are a feature of the [Table of Contents][] extension, which is part
+of the standard Markdown library. The extension inserts a link at the end of
+each headline, which makes it possible to directly link to a subpart of the
+document.
+
+## Installation
+
+To enable Permalinks, add the following to your `mkdocs.yml`:
+
+``` markdown
+- toc:
+    permalink: true
+```
+
+This will add a link containing the paragraph symbol "¶" at the end of each
+headline (exactly like on the page you're currently viewing), which the
+Material theme will make appear on hover. In order to change the text of the
+permalink, a string can be passed, e.g.:
+
+``` markdown
+- toc:
+    permalink: "Permalink"
+```
+
+## Usage
+
+When enabled, permalinks are inserted automatically.
+
+[Table of Contents]: https://pythonhosted.org/Markdown/extensions/toc.html