`&`-s not escaped in search results

[mortenpi]

I think we directly pass the &-s to the generated dynamically generated HTML, which means they get interpreted as character references sometimes. We should escape them, either when constructing the HTML, or maybe even during index generation (although I am not sure this actually makes sense).

X-ref for more details for an "MWE": JuliaLang/julia#52078

[fingolfin]

I was confused by this issue at first, because I thought this was about & occurring in the search string entered by the user. But really this is about & occurring in the data being searched. The example from PR #2461 is instructive: insert

### ↔ ↔ ↔

into the Markdown source (then ↔ is rendered as ↔), then search for the text " harr" -- it will match those arrows.