Cleanup example catalog

[niktayv]

Contributes to TerriaJS/terriajs#7521

1. I've removed the items with dead and unrecoverable URLs (see the commits for the details).
2. I've tested that all the remaining items are working correctly (or made them work by fixing the URLs).
3. I've introduced two groups: Featured Examples and Basic Examples and moved the items to those groups according to my understanding.
4. I plan to introduce one more group, Advanced Examples, where the users will have to add their API keys and other configuration options for the items to work correctly.

Now, the default catalog upon opening looks as follows:

Test it here: http://ci.terria.io/main/#clean&https://raw.githubusercontent.com/TerriaJS/TerriaMap/6c41c9f60553daeafd9c76a449b4df9dd174bc8d/wwwroot/init/simple.json

[micholm]

LGTM thanks!