Docs: clarify Web packaging requirements in developer-setup vs supported-platforms

**Forum Post:** https://community.cesium.com/t/adding-webgl-specific-packaging-requirements-to-developer-setup-md/46720
**Category:** Cesium for Unity
**Reported by:** @hv_nico (community forum)
**Cesium/Unity versions mentioned:** Unity 6+ for Web, current docs still recommend Unity 2022.3 for developer setup

## Problem

`Documentation~/developer-setup.md` currently reads as a general prerequisite list and still recommends Unity 2022.3 LTS, while Web support requires Unity 6+ and additional Web-specific setup documented elsewhere. This creates confusion for users trying to package for Web.

The forum report also points out that WebGL native build configuration uses `-G Ninja` in `Source/Editor/CompileCesiumForUnityNative.cs`, so tooling expectations are not obvious from contributor docs.

## Proposed docs updates

1. In `developer-setup.md`, explicitly scope Unity 2022.3 guidance to general contributor/editor workflows.
2. Add a “Web packaging prerequisites” subsection that links to `supported-platforms.md#web`.
3. Explicitly call out Unity 6+ for Web target builds.
4. Add a note about Web build tooling expectations (including Ninja-related behavior and where to troubleshoot).
5. Add cross-links between `developer-setup.md` and `supported-platforms.md#web` to avoid drift.

## Source references

- https://community.cesium.com/t/adding-webgl-specific-packaging-requirements-to-developer-setup-md/46720
- https://cesium.com/learn/cesium-unity/ref-doc/developer-setup.html
- https://cesium.com/learn/cesium-unity/ref-doc/supported-platforms.html#web
- https://github.com/CesiumGS/cesium-unity/blob/main/Source/Editor/CompileCesiumForUnityNative.cs#L619-L626
- https://github.com/CesiumGS/cesium-unity/blob/17ba7d50dfdc533ce4338162cc13fd47ad45986a/.github/workflows/build.yml#L490-L492
- https://cesium.com/blog/2026/03/16/introducing-cesium-for-unity-web/

Read Documentation~/developer-setup.md and supported-platforms.md#web first, then inspect Source/Editor/CompileCesiumForUnityNative.cs around lines 619-626 and the referenced build workflow lines. Clarify that Unity 2022.3 applies to general contributor workflows, document Unity 6+ and Web packaging prerequisites, explain the Ninja-related tooling expectation, and add reciprocal links without duplicating platform details.

Written by the indexing model from the issue text.