Build a mkdocs plugin for multi-repo docs generation

[ptbrowne]

At the moment, the multi doc pulling and consolidating is done in an adhoc way. Other people are searching for a multi docs plugin for mkdocs.

1. generate_docs could be made into a mkdocs plugin using the on_config event to regenerate the config
2. OUTSIDE_DOCS file could be put inside the plugin configuration

mkdocs/mkdocs#265

[jdoiro3]

See my comment here.