🧹 v3 (Maintenance): Update docs to reflect fiber.Ctx struct to interf…

…ace change (#2880)

* 🧹 [v3 Maintenance]: Update docs to reflect fiber.Ctx struct to interface change

Summary:
- Update `Static.Next()` in `/docs/api/app.md` to use the `Ctx` interface
- Update `/docs/api/ctx.md` to use the `Ctx` interface

Related Issues:
#2879

* 🧹 [v3 Maintenance]: Update Ctx struct description to interface

Related Issues:
#2879