Update the documentation [skip ci]

* Update the "experimental" description of the "Fix for nested lists
  rendering in Material" feature as it's now in the `master` branch.

README.md

@@ -199,7 +199,7 @@ markdown_extensions:
     - toc:
         permalink: true
 ```
-## Experimental features
+## Custom features
 
 ### Automatically generating the MkDocs configuration
 
@@ -207,7 +207,7 @@ For usage of the `mkdocs_configuration` CLI please run `docker run -i -t gitlab-
 
 ### Better rendering of nested lists for the Material theme
 
-The `mkdocs_material_nested_lists_rendering_fix` script is available in the `gitlab-registry.cern.ch/authoring/documentation/mkdocs:dev_material_nested_lists_rendering` docker image. It addresses the issue described [here](https://github.com/squidfunk/mkdocs-material/issues/508). To use it just add `- mkdocs_material_nested_lists_rendering_fix` under `script` in the `building` stage in your CI configuration (`.gitlab-ci.yml` file) right before `- mkdocs build --clean --site-dir public`.
+The `mkdocs_material_nested_lists_rendering_fix` CLI addresses the issue described [here](https://github.com/squidfunk/mkdocs-material/issues/508). To use it just add `- mkdocs_material_nested_lists_rendering_fix` under `script` in the `building` stage in your CI configuration (`.gitlab-ci.yml` file) right before `- mkdocs build --clean --site-dir public`.
 
 ## Contributing