Clean up default rosdoc2.yaml to show all available options, but commented out.

[rkent]

Best practice IMHO in a default configuration file should be to show all available options without requiring consulting documentation or source code, with the options showing default values but commented out. That way a user can easily modify the default file by uncommenting and changing a single option. It is then clear what was changed.

This PR cleans up the default rosdoc2.yaml file according to those principles. It is done in a series of changes that could be reviewed separately, but they are all related by a single goal.

The changes are:

• remove the unused generate_package_index
• comment out "settings" section options
• comment out "builders" section options
• add any missing options
• add a test that re-enable the commented out sections to show that the defaults really do work.

The test is just a copy of the full_package tests, but with a default rosdoc2.yaml with defaults uncommented out. The test has one uncommented item, sphinx_sourcedir. This cannot be uncommented until #137 is fixed.

This PR is motivated by cleaning up some of the configuration in preparation to a more complete documentation of the customization options.

[tfoote]

Thanks for this cleanup. It's definitely going to provide a better user experience.