Automatically run sphinx-apidoc during documentation build

At the moment, the steps involved in generating documentation for most ETS projects are

- run sphinx-apidoc to generate the api documetation
- run sphinx-build to build the docs

This is problematic for the RtD docs setup for ETS projects as at the moment as it simply runs `sphinx-build` by default. We could potentially use the RtD config file to specify the additional `sphinx-apidoc` step but it might be easier to take this approach in traits-futures - https://github.com/enthought/traits-futures/pull/348 - where `sphinx-apidoc` is run as part of `sphinx-build`.

This also simplifies the documentation build process in general.

Note that because of this issue, none of the ETS projects on RtD which use autogenerated API docs have API documentation.

- [ ] apptools
- [ ] enable
- [ ] envisage
- [ ] pyface
- [ ] traitsui