Update Search http API documentation

[mbenadda]

Bug Report

Expected behavior/code API docs (Blueprints https://github.com/openfun/richie/blob/master/src/richie/apps/search/description.apib) should be up-to-date with the actual API.

Actual Behavior The docs are missing critical parts of the API, such as facets.

Steps to Reproduce N/A

Environment N/A

Possible Solution Update the API docs to match the current state of the API

Additional context/Screenshots N/A

[jmaupetit]

Maybe adding automated testing with Dredd in the CI would help keeping this spec up-to-date?