Redirect Python API documentation to new Sphinx-generated pages

With 5.1.0-m1 the epydoc has been deprecated from the build so the old API
documentation links are obsolete and need to be redirected to the new
Sphinx-generated API documentation. A specific extlink is added to point at
this Python-generated documentation.
Merge pull request #1011 from sbesson/sphinx-api

Redirect Python API documentation to new Sphinx-generated pages