Currently, the web api is documented in two different places:

• directly in the Python code through the use of decorators from the apidoc module
• in swh-web sphinx documentation, using the httpdomain extension

We should remove that documentation duplication and only rely on using sphinx and
httpdomain. The current sphinx documentation should be spread in relevant docstrings
of the api endpoints implementation. The apidoc module will now parse those docstrings
to extract the relevant info (endpoint description, url arguments and query parameters,
desciption of the json object returned, ...).