docs(router): align RouteMatch API reference with the current public contract

Goal

Bring the RouteMatch API reference in line with the current public contract.

Scope

• Remove obsolete showPending and fetchedAt entries from docs/router/api/router/RouteMatchType.md.
• Add fullPath using its route-specific type, Route['fullPath'].
• Add index and explain its ordering within the matched route array.
• Update cause to include 'preload'.
• Keep internal and lifecycle-specific match state out of the reference, including displayPendingPromise, fetchCount, preload, invalid, headers, and globalNotFound.
• Review the complete page against packages/router-core/src/Matches.ts so the reference describes the intentional public API.

Related Work

This continues the RouteMatch reference update started in #4894

Start with docs/router/api/router/RouteMatchType.md and compare it against packages/router-core/src/Matches.ts, using the public contract described in the issue as the guide. Remove obsolete and internal fields, add the specified public fields and cause value, and review the complete page for accuracy. Done means the reference matches the intentional public RouteMatch API without lifecycle-only state.