docs: modernize README and improve npm discoverability

[willeastcott]

Summary

Modernize the README and package metadata to improve npm discoverability and better reflect the engine's current capabilities.

package.json

• Update description from "PlayCanvas WebGL game engine" to "Open-source WebGL/WebGPU 3D engine for the web"
• Add high-value npm search keywords: 3d-engine, game-engine, gaussian-splatting, physics, renderer, scene-graph, ecs

README.md (all languages)

• Add Install section with npm install playcanvas and npm create playcanvas@latest
• Replace deprecated model component with render in the usage example
• Switch from namespace import (import * as pc) to named imports for tree-shaking
• Add Gaussian Splatting to the Features list
• Update Graphics feature to reference WebGL2 & WebGPU (was WebGL 1.0 & 2.0 in JA/KR)
• Add Ecosystem section linking to @playcanvas/react, @playcanvas/web-components, create-playcanvas
• Reorder sections: Install, Usage, Features, Ecosystem, Showcase, Users, Build, Editor
• Fix Korean README language links from master to dev branch

[Copilot]

Pull request overview

Modernizes the project READMEs and npm package metadata to better reflect current engine capabilities and improve npm discoverability.

Changes:

• Updated package.json description and expanded npm keywords for better search relevance.
• Reworked README structure across languages (Install/Usage/Features/Ecosystem ordering) and refreshed the usage snippet (named imports + render component).
• Added Gaussian Splatting to Features and introduced an Ecosystem section linking related packages.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
README.md	Adds Install + Ecosystem sections, updates usage example to named imports and render component, refreshes features list.
README-zh.md	Aligns structure/content with English README; adds Install/Features/Ecosystem sections and updates usage example.
README-kr.md	Aligns structure/content with English README; updates language links to dev, adds Install/Features/Ecosystem, updates usage example.
README-ja.md	Aligns structure/content with English README; adds Install/Features/Ecosystem sections and updates usage example.
package.json	Updates package description and keywords to improve npm discoverability.

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