Display non-empty page TOC

[chalin]

Proposes a solution to #853 along the lines of #853 (comment):

... show the TOC if and only if it is non-empty. If projects want to refine this behavior (by counting the number of TOC entries for example), they can override the template.

This is an alternative proposal to #1110 which doesn't rely on the (IMHO) faulty TOC character-count metric.

Preview:

• https://deploy-preview-1147--docsydocs.netlify.app/docs/best-practices/ - no TOC
• https://deploy-preview-1147--docsydocs.netlify.app/docs/best-practices/site-guidance/ - TOC with a single entry (which previously would have been hidden)
  • Compare with: https://www.docsy.dev/docs/best-practices/site-guidance/
• https://deploy-preview-1147--docsydocs.netlify.app/docs/ - an example of a TOC that would have been shown before as well.

So if y'all agree, this would be a step forward:

• Fixes Page TOC not showing up #853
• Closes parameter to customize the length of toc #326
• Closes Configurable TOC limit with default 200 #1110 by superseding it

[LisaFC]

LGTM!

[polarweasel]

Elegant solution. Thank you @chalin!