(#267) Don’t show “Public X” header without contents

[danielparks]

Previously, the “Public X” header would be displayed if there were any private X regardless of whether there were any public X.

For example, having a private function would make both the “Public Functions” and “Private Functions” headers appear, even if there weren’t any public functions.

This changes it so that there are three conditions:

• Only public X: no “Public” or “Private“ headers are displayed; the X are listed with links to their documentation. (Public is implied.)
• Only private X: a “Private X” header is is displayed with a list of X. There are no links to documentation for private APIs.
• Both public and private X: both “Public X” and “Private X” headers are displayed. The public Xs are listed with links to their documentation; the private Xs are just listed with no links.

In other words, if there are no private Xs, then it just lists the public once. Otherwise, it splits them up under public/private headers but avoids showing a header if it’s contents will be empty.

This also radically simplifies and removes a bunch of boilerplate code around rendering these sections.

[danielparks]

I realized that it was a lot clearer to pull PuppetStrings::Markdown::TableOfContents into the general renderer, so I did. I also did some future-rubocop-related clean ups, though it’s still going to take some work to rebase this after #310 lands.