Skip empty Contents section

[izmalk]

• In a very specific case when our docs don't have multiple pages and the Contents section is empty we should not add the Contents heading because it has no content.
• Also, the heading in this case is added as the first level heading which is equivalent to the Documents title. The best practice is to have only one first-level heading - at the very beginning of the document.

I think we should never be able to generate the first-level heading for anything except the page's title. And we should skip the Contents heading if it has nothing inside.

Example

canonical/karapace-operator#15 -- In this PR we add simple documentation to a product, consisting of a single page.

See the Discourse topic for the documentation.