Add a link to the dashboard source code

[bajtos]

Make it easier to navigate from the deployed dashboard to the source code where the dashboard is defined.

Dashboard	Source
https://dashboard.filspark.com	https://github.com/filecoin-station/spark-dashboard/blob/main/src/index.md
https://dashboard.filspark.com/provider/f03156617	https://github.com/filecoin-station/spark-dashboard/blob/main/src/provider/%5Bprovider%5D.md

For example, we can add a link to the footer.

- Built with [Observable](https://observablehq.com/) on Jan 5, 2025.
+ Built with [Observable](https://observablehq.com/) on Jan 5, 2025 ([source code](https://github.com/filecoin-station/spark-dashboard/blob/main/src/index.md)).