Updating the documentation of BioImage.IO

Add a new item to the sidebar

The item (markdown) lives in docs

• Add the new item in bioimage.io/docs/_sidebar.md. E.g.:

  * [Getting Started](getting_started.md)
  

  Documentation is rendered from the docs folder, so the markdown needs to live there.

The item (markdown) lives in another repository

• Add the alias item into the index.html file. For example:

        homepage: homepage,
      alias: {
        '/bioimageio_spec': 'https://raw.githubusercontent.com/bioimage-io/spec-bioimage-io/master/README.md'
      }
  

• Add the new item in bioimage.io/docs/_sidebar.md. E.g.:

  * [Contribute Models](/contribute_models/README.md)
    - [Model Resource Description File (RDF)](https://bioimage.io/docs/#/bioimageio_model_spec)