Improve naming and description of Sandbox Explorer STAC catalogue

[robbibt]

Issue

Currently the Sandbox Explorer provides the STAC endpoint to the production database for DEA data on AWS: https://explorer.sandbox.dea.ga.gov.au/stac/

Matthias Mohr recently added this endpoint to the official STAC index (https://stacindex.org/catalogs/digital-earth-au#/), and pointed out that the STAC catalogue metadata could be improved: https://twitter.com/matthmohr/status/1417127562787045387

In particular, the "id" and "title" STAC catalogue is "Sandbox Explorer", which doesn't make it clear that this is the main STAC metadata for the Digital Earth Australia program. In addition, the "description" is generic and uninformative:

"id": "Sandbox_Explorer",
"title": "Sandbox Explorer",
"description": "Configure stac endpoint information in your Explorer `settings.env.py` file"

Suggested fix

As this is our main STAC endpoint, we should rename the "id" and "title" to better portray this as the place to access metadata about DEA's data. We should also improve the description field to better describe the catalogue. For example:

"id": "digitalearthau",
"title": "Digital Earth Australia",
"description": "Digital Earth Australia (DEA) is a platform that uses spatial data and images recorded by satellites orbiting our planet to detect physical changes across Australia in unprecedented detail."

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