api: Remove duplicated documentation and improve it

Previously, the web api was 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

The documentation now only relies on using sphinx and httpdomain.
Sphinx documentation have been spread in relevant docstrings of the api
endpoints implementation and the apidoc module now parse those docstrings
to extract the relevant info (endpoint description, url arguments and
query parameters, desciption of the json object returned, ...).

Web api HTML interface has also been improved in terms of semantics
and design.

Closes T1105