fix(html): correctly handle examples in document navigation

[crowlKats]

Fixes denoland/docs#573.

Fixes 3 things:

• if first element is not the top-most element in a document navigation, it would break since it would only look at the first element's depth, instead of taking the highest ranking element's depth.
• removes the top-level rendering of "Examples" in the document navigation for functions, since functions have examples in each overload instead of at the generic symbol level.
• adds support for individual example titles being rendered in the document navigation, unlike before where it would only render "Examples" without any sub-elements.

[bartlomieju]

LGTM