docs(router): show route-bound context access in a route component

[LadyBluenotes]

Goal

Show how a route component reads its merged router context.

Scope

• Add a component example to docs/router/guide/router-context.md.
• Use Route.useRouteContext() for the route-bound case.
• Keep the example consistent with the preceding Route declaration instead of redefining it.
• Explain or link to the standalone useRouteContext({ from }) API for components that need context from another route.
• Keep the guide compatible with the shared Router documentation structure.

Related Work

This continues the documentation update started in #5022

[soheilnikroo]

@LadyBluenotes Added a component example to docs/router/guide/router-context.md under Using the Router Context:

Extends the existing /todos route with a Todos component using Route.useRouteContext()
Explains route-bound access and narrowed TypeScript inference
Shows useRouteContext({ from: rootRouteId }) for reading context from another route, with a link to the API reference