Docs: Improve documentation around GeoJSON

[tordans]

References https://osmus.slack.com/archives/C04M1NHPEQY/p1740551156419729, https://osmus.slack.com/archives/C04M1NHPEQY/p1740582840392009

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I propose to add to the docs about how the GeoJSON for MR can and should be defined and use this opportunity to restructure some docs around GeoJSON.

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Suggestion:

• Create a dedicated entry page that explains the supported and explicitly the recommended format for uploading date to MR and for the format that APIs should return when using the "external URL" option.
• Given the structure of the navigation, it might be easiest to set up a headline "GeoJSON" and add all the pages above in this section
• Most of existing pages could mostly stay as is but be linked from that central page – and link back to it as an introduction sentence that ties it together.

Status quo:

Title	URL	Comment
Line-by-Line GeoJSON Format	documentation/line-by-line-geojson/	Clarify the deviation from the GeoJSONL standard: MR requires FeatureCollections, not Features; Maybe mention lacking support in QGIS/Placemark and ideally how to test this MR format in QGIS software
Create a Challenge with GeoJSON data	screencasts/create-a-challenge-with-geojson-data/	Did not watch the screencast but I think it uses the cli which should be mentioned on the page
Hosting Remote GeoJSON with Gists	/documentation/gists-for-remote-geojson/	This page should link back to explain the format of the GeoJSON in the Gist
Styling Task Features	/documentation/styling-task-features/	Extract the part on GeoJSON to the new page that is linked in the GeoJSON section
Task Attachments	/documentation/task-attachments/	Don't know what that is/does but should be part of the the new GeoJSON section as well