Create initial GitLab CRD documentation

For the general availability releases we need to have end user documentation for the GitLab CRD.

Documentation should include and overview of the CRD structure and an itemized list of attributes used in the CRD with their descriptions.

Not sure if we use any tools to generate the API documentation that could be used for pulling documentation out of the CRD files, but that would probably be preferable instead of manually updating the documentation.