docs: add BirdEcho mobile companion app to Cool Links

[arunrajiah]

What this changes

Adds a Mobile Apps subsection under Cool Links with a one-line entry for BirdEcho — an open-source mobile companion app for BirdNET-Pi stations.

Proposed addition

### Mobile Apps
- [BirdEcho](https://github.com/arunrajiah/birdecho) — open-source Android/iOS companion app. Connects directly to BirdNET-Go stations over your local network (no cloud account needed), or via the BirdWeather API for BirdNET-Pi and BirdWeather stations. Live detection feed, audio playback, species browser, 14-day chart, and local notifications for favourite species. MIT licensed.

Notes

• BirdNET-Pi users: BirdEcho connects via the BirdWeather API, so BirdWeather integration must be enabled on the station (Tools → Settings → BirdWeather).
• BirdNET-Go users (as of v0.2.0): BirdEcho can connect directly over the local network with no BirdWeather account needed.
• It is not affiliated with or endorsed by the BirdNET-Pi project.
• Happy to adjust wording or placement to match the project's conventions.