Reactive the viewcode extension for docs (Cherry-pick of #1312) (#1315)

We got source links working in API docs on docs.quantum.ibm.com! See Qiskit/documentation#620. To do this, we need to active `sphinx.ext.viewcode`.

Note that we still plan to improve this implementation in the future to have more precise links by using `sphinx.ext.linkcode`, tracked by Qiskit/documentation#517. This PR is to unblock the first iteration of this source links mechanism.

docs/conf.py

@@ -33,6 +33,8 @@
     'sphinx.ext.napoleon',
     'sphinx.ext.autodoc',
     'sphinx.ext.autosummary',
+    # This is used by qiskit/documentation to generate links to github.com.
+    "sphinx.ext.viewcode",
     'sphinx.ext.mathjax',
     'jupyter_sphinx',
     'sphinx_autodoc_typehints',