Add API documentation for VulnerableCode.io

[mjherzog]

We need to add basic API usage documentation for VulnerableCode.io. This should probably be added in the Reference section and then we can provide a direct link to the new section from VCIO. My assumption is that this documentation can be generated in the same way as we handle API documentation for SCIO

[TG1999]

We can use either redoc or swagger-ui https://www.django-rest-framework.org/topics/documenting-your-api/

[TG1999]

We were using drf-spectacular previously, but it's not been used anymore because of tfranzel/drf-spectacular#389

[pombredanne]

This is completed. Closing. See https://public.vulnerablecode.io/api/docs/