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

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

Start with docs/router/guide/router-context.md and inspect the preceding Route declaration and the shared Router documentation structure. Add the route-component example using Route.useRouteContext(), then explain or link to useRouteContext({ from }) for another route; the guide is done when both access patterns are clear and the existing declaration is reused.