As mentioned in goquery's documentation (https://pkg.go.dev/github.com/PuerkitoBio/goquery#Single):
> By default, Selection.Find and other functions that accept a selector string
to select nodes will use all matches corresponding to that selector. By using
the Matcher returned by Single, at most the first match will be selected.
>
> The one using Single is optimized to be potentially much faster on large documents.
- make findContentUsingCustomRules' more idiomatic,
since in golang a function returning an error might
return garbage in other parameter. Moreover, ignoring
errors is bad practise.
- getPredefinedScraperRules is now running in constant-time,
instead of iterating on a list with around 50 items in it.
