docs: elevate the Entity class reference to peak quality

[willeastcott]

Description

First page in a page-by-page effort to bring flagship API reference pages up to the standard already set by the best pages in the engine (e.g. GSplatComponent, Vec3). Entity is the pilot that locks the bar for the rest.

What changed

• Rewrote the Entity class overview into an authored introduction rather than a bare definition:
  • the mental model — an Entity is a named node in the scene graph that gains behavior from the {@link Component}s attached to it;
  • the two facets it combines — a transform (from {@link GraphNode}) and a set of components;
  • the typical workflow — addComponent → access via the matching property → removeComponent, plus group enable/disable via GraphNode#enabled and removal via destroy;
  • tight cross-links to Component, GraphNode, AppBase#root, and the common components.
• Added two runnable class-level @examples (create a camera entity and add it to the scene; parent/child transform inheritance).
• Accuracy fix: findComponent() was typed {Component} but documents and can return undefined → now {Component|undefined}.

Deliberately unchanged: the 21 component accessors stay terse (they already link their component type), and the other methods (addComponent, destroy, clone, …) were already at the bar.

Verification

• npm run lint ✅
• npm run docs — 0 errors, no new warnings; all new {@link}s resolve; overview renders as a bulleted list with both examples ✅
• npm run build:types + npm run test:types ✅

Checklist

• I have read the contributing guidelines
• My code follows the project's coding standards
• This PR focuses on a single change

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR improves the Entity API reference by replacing the terse class overview with a more complete conceptual introduction and examples, while also updating findComponent() documentation.

Changes:

• Expanded Entity class-level JSDoc with workflow guidance and cross-links.
• Added two class-level examples for camera creation and hierarchy transforms.
• Updated findComponent() return documentation.

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