Add toc_sticky parameter's description (mmistakes#2741)

docs/_docs/10-layouts.md

@@ -129,6 +129,7 @@ Auto-generated table of contents list for your posts and pages can be enabled by
 | **toc**     | Optional | Show table of contents. (boolean) | `false` |
 | **toc_label** | Optional | Table of contents title. (string) | `toc_label` in UI Text data file. |
 | **toc_icon**  | Optional | Table of contents icon, displays before the title. (string) | [Font Awesome](https://fontawesome.com/icons?d=gallery&s=solid&m=free) <i class="fas fa-file-alt"></i> **file-alt** icon. Other FA icons can be used instead. |
+| **toc_sticky** | Optional | Stick table of contents to top of screen.                   | `false` |  
 
 **TOC example with custom title and icon**