Fix OpenSeaSample and Sample Documentation

[inforithmics]

Please summarize your PR and explain which problem it solves.

1. Fixed OpenSea Sample (use another Webservice)
2. Add Gebco_2021 Sample (the opensea sample used an gebco_2021 Image).
3. Fixed missing Samples Documentation.

[pauldendulk]

Somewhere else I use the open tile service pasted below, did you look into that? It seems to behave fine.

private static HttpTileSource CreateTileSource(string userAgent)
{
    return new HttpTileSource(new GlobalSphericalMercator(),
        "https://tiles.openseamap.org/seamark/{z}/{x}/{y}.png",
        new[] { "a", "b", "c" }, name: "OpenSeaMap",
        userAgent: userAgent, persistentCache: DefaultCache);
}

[inforithmics]

Somewhere else I use the open tile service pasted below, did you look into that? It seems to behave fine.

private static HttpTileSource CreateTileSource(string userAgent)
{
    return new HttpTileSource(new GlobalSphericalMercator(),
        "https://tiles.openseamap.org/seamark/{z}/{x}/{y}.png",
        new[] { "a", "b", "c" }, name: "OpenSeaMap",
        userAgent: userAgent, persistentCache: DefaultCache);
}

This shouldn't be a problem the problem was that this service
https://depth.openseamap.org/geoserver/gebco2021/wms disappeared.
So I use now this web service to test opensea.
https://depth.openseamap.org/geoserver/ows

Because the the disappeared service was gebco2021 service I added a gebco 2021 service sample.

[pauldendulk]

I will merge this if it is green.

[inforithmics]

To fix build Uno needs to be updated. Because the Uno GPU Pull request is green and there I upgraded Uno.

[pauldendulk]

Thanks