Generate hash for index.json requests (FSharp.Formatting issue)

The index.json search index cached by browsers may become out of date as the site gets refreshed

This is an FSharp.Formatting issue (specifically in fsdocs-search.js) noted by @baronfel in https://github.com/fsharp/fsharp-core-docs/pull/27#issuecomment-667561113

> > Cool. How do we check that? Here's the request: fsprojects/FSharp.Formatting@master/docs/content/fsdocs-search.js#L14
>
> Regarding this point, I'd suggest that that content rendered by the templating engine render the content, and provide a hashed value for this specific content
>
> Then, when rendering links/src elements to generated content files, append the hash to the query string. This would mean that urls like the one in the search JS you linked would need to be templated as well. Same for css links in the html head elements, etc.