Web app: Remove duplicated api documentation

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, ...).

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Migrated from T1105 (view on Phabricator)