"/packs/:id/gallery/:index" endpoint is undocumented

The `/packs/:id/gallery/:index` endpoint is not mentioned anywhere in the documentation, yet it's the preferred method of obtaining pack gallery images **used by the main website as well**.

I got the URL schema from [src/routes/packs/id/index.ts](https://github.com/Smithed-MC/UX/blob/26e8536a615ef96ca37eb2d67928c6a7b5444681/platforms/api/src/routes/packs/id/index.ts#L699).