Prepare 1.13.0 (#480)

Three projects are now using the new theme in production:

- https://qiskit.github.io/qiskit_sphinx_theme/
- https://qiskit-extensions.github.io/circuit-knitting-toolbox/
- https://qiskit-extensions.github.io/quantum-serverless/

Rustworkx is ready to switch after landing a PR to reorganize their
docs.

While some projects will still need to reorganize their API docs to be
able to switch to the new theme, there are no changes we expect
necessary to qiskit-sphinx-theme itself. They can speed up prod builds
by reorganizing their API docs, and they can speed up their dev builds
even more by using `sphinx-remove-toctrees`.

If these approaches prove not feasible for the slow docs build, then we
can do a qiskit-sphinx-theme 1.14 release with changes to
qiskit-sphinx-theme. We are not yet deleting the Pytorch theme. In the
meantime, other projects can start switching now.

src/qiskit_sphinx_theme/__init__.py

@@ -22,7 +22,7 @@
     import sphinx.application
     import sphinx.config
 
-__version__ = "1.13.0rc2"
+__version__ = "1.13.0"
 __version_full__ = __version__