doc: Added plugins as ToC elements instead of orphaned links

  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.