o Now the page titles are declared in plugins, allowing for
    a more descriptive ToC

  o Makefile and plugin.rsttemplate updated to not produce the title,
    to no longer use `:orphan:` for plugin pages, and to ignore any
    private modules in the plugin directories.

  o Interestingly, now the docs will fail to build if you add
    a new plugin and forget to add it to the documentation.

Some of the warnings from sphinx-build are really just warnings,
but a lot of the things we want to avoid and really break documentation,
like broken internal references and some invalid rst directives should
really be errors.

Now we treat all warnings as errors, this should ensure that
any commits landing upstream never break the docs.

This makes the 'sources' and 'elements' subdirectores modules technically,
but it does not effect how we load them with pluginbase, that still works.

Updated documentation machinery to have buildstream/plugins in the PYTHONPATH
and import the docs as elements.autotools etc.

This is all because since recent sphinx started importing from distutils,
this was conflicting with our distutils plugin.

Replace sphinx-apidoc for plugin API docs generation.