HTML search: allow grouping results from the same page

[neongreen-sc]

https://readthedocs-sphinx-search.readthedocs.io/en/latest/?rtd_search=testing

RTD search shows section headings from pages. For example, if something is found in the "Development" and "Testing" headings in a document, RTD shows both and allows jumping directly to the heading.

This is very useful for long pages like glossaries, lists, etc.