Simplify process to create dataset-specific openapi docs

[thierrydallacroce]

This PR proposes a refactoring of the WebServiceApiDocs class, where a couple things seemed awkward:

1. Having to know the complete list of paths and operations we did not want. Adding new paths to our API would break the dataset-specific doc or require these new paths to be removed as well, which seemed repetitive. We simplify the logic by flipping it: chose only what to keep, discarding anything else.
2. API paths and operations where treated and removed individually. It removed a verb everywhere, without allowing to keep a verb in one path but not another. Now paths and verbs are treated as a unit to describe one endpoint. More combinations are possible.