As an aside, Scrivener already supports very basic indexing, which is probably about like what you’d get in a word processor—you can mark index keys, or flag readable words as keys. It involves using Scrivener’s Markdown compile integration, which the post walks you through setting up if you’ve never used that before. With text that uses styles for all non-paragraph formatting, it can be used without much general awareness of Markdown too, if that isn’t something you’re personally interested in using.
But as with word processors, it is pretty basic and will not offer some of the more sophisticated and useful features a dedicated package will. I’ve never used one myself, so I can’t offer any tips there, but figured that it’s good to note that for those with basic needs, you aren’t stuck compiling and then doing all of the indexing on the final copy, which would essentially mean abandoning the Scrivener project given how much work that would entail.