docs: Add a sentence or two to explain what Shorebird is.

**Description** Neither the docs site nor the main repo's README mention the most basic info about what Shorebird is, nor what it does. **Requirements / Suggestion** - [ ] Add something like this to the docs.shorebird.dev home page (`/src/content/docs/index.mdx`): Shorebird is a hosted “code push” / over‑the‑air (OTA) update platform specifically for Flutter apps. It lets you ship updated Dart code to already-installed apps on iOS and Android without going through a full App Store / Play Store review and binary release each time. (Description taken from Perplexity)