Automatically run sphinx-apidoc during documentation build

[rahulporuri]

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

[mdickinson]

From a quick regex search, the affected repositories look like: apptools, enable, traitsui, pyface and envisage.