docs: Add transient identities and traits to the client-side JS SDK page

The [client-side JS SDK docs page](https://docs.flagsmith.com/integrating-with-flagsmith/sdks/client-side-sdks/javascript) does not cover transient identities or traits. Users discovering the feature currently have to reach for the [JS SDK changelog](https://github.com/Flagsmith/flagsmith-js-client/blob/main/CHANGELOG.md#version-500---support-context-transient-identities-and-traits---2024-10-02) to find it.

Transient identity/trait support shipped in JS SDK v5.0.0 (Oct 2024) but has not been reflected on the docs page since.

### Scope

- New section on the JS SDK page explaining transient identities and traits
- Init example using a transient identity (e.g. `UUIDv4` + traits)
- Guidance on when to use transient vs persistent identity for common client-side cases (localization, geo, UX variations, ephemeral user context)
- Cross-reference the Evaluation Context schema