De-unify picking/shadows/custom-shaders; add GLSL/WGSL tabs

[mvaligursky]

Continues the Gaussian splat docs cleanup now that unified rendering is the only mode, and improves the shader code blocks.

De-unify (remove non-unified content and "unified mode" framing):

• Picking — removed the "Unified vs Non-Unified Mode" section and the non-unified complete example; the page now plainly states the picker returns the GSplatComponent (and enableIds is just setup).
• Shadows — the "Shadow Quality" section now sets app.scene.gsplat.alphaClip on the global GSplat settings; removed the Unified/Non-Unified subsections.
• Custom Shaders — rewritten around the global scene gsplat material (app.scene.gsplat.material) and the gsplatModifyVS chunk. Kept the (now unified) multi-splat example and added a "How the Example Works" walkthrough; removed the non-unified per-component material content and the redundant helper/example sections.

Shader code blocks:

• Custom Shaders and Splat Data Format now present GLSL/WGSL as synced tabs (shared groupId="shader-language" + queryString="lang", so the choice syncs across pages and is URL-shareable).
• Registered the wgsl Prism language in docusaurus.config.js so WGSL blocks get syntax highlighting (previously only GLSL did).

Mirrored across the Japanese (i18n/ja) docs.

Note: docusaurus.config.js changed, so a dev-server restart is needed locally to pick up the WGSL highlighting.