ReadTheDocs version warning MkDocs plugin

"ReadTheDocs version warning MkDocs plugin" is a very basic mkdocs plugin that replace placeholder text

### VERSION-WARNING-BANNER-PLACEHOLDER ###

in the theme template with warning admonition.

Configuration

plugins:
    - readthedocs-version-warning:
        project_id: "000000"
        show_on_versions:
            - latest

project_id - Id of the target project on ReadTheDocs. From this project will be used the default version to render link to the stable version.

show_on_versions - List of versions on which the banner should appear.