Create reStructuredText directives that build the module and package toctrees for pipelines.lsst.io.

Building these toctrees automatically will make the pipelines_lsst_io repo more robust to a changing composition of stack packages.

The list of modules and packages available for the documentation should generally be known from eups setups and the presence of manifest.yaml files.

The directives should allow for packages to be explicitly ignored even if they are automatically detected. This feature should be easy to implement and will provide a safety valve for automatic package detection.

The directives will be implemented in https://github.com/lsst-sqre/documenteer.