Remove un-needed slugs

[lornajane]

We use page slugs in front matter in many pages of the project, most seem to be the result of copy/pasting existing pages when creating new ones. These should be removed wherever possible since the absolute address makes it more difficult to preview the documentation within the CLI repo itself - the slugs are designed to work only as part of the combined documentation set at https://redocly.com/docs/cli

The advantage of the slug is to support camel case in a URL (where by default we lowercase it), but the cost of more difficult documentation maintenance doesn't seem worth it so let's remove these.