Pages with non-trivial but short TOCs don't get displayed

[chalin]

The Data sources page has three H2 headings but no TOC is displayed in the right nav:

This is because the current toc.html partial (somewhat arbitrarily IMHO) decides whether it should display the TOC based on the character length of the TOC HTML! We can do better than this: e.g., at least count the number of headings.

Info:

• https://gohugo.io/content-management/toc/
• parameter to customize the length of toc google/docsy#326