docs: improve sphinxcontrib-openapi rendering

[diegodelemos]

The current output for a given OpenAPI spec using sphinxcontrib-openapi is very poor for the moment (see REANA Job Controller API).

We are looking for:

• [ ] Display all the useful information contained in the specification
• [ ] Structure the information in a meaningful way
• [ ] Provide an option to select which HTTP method the user want to display for a given endpoint (currently there is only available an option to select the endpoint, so all the methods for the given endpoint are displayed, see here)
• [ ] Submit a PR with tests and docs upstream sphinxcontrib-openapi

Some sources of inspiration:

• ClaimStore
• GitHub
• Context.IO

There is some code here, which can serve as a starting point. This would be the output:

[tiborsimko]

This could be attacked in another, follow-up milestone.