Introduce 3D Globe view

[DelazJ]

There is no issue report 😠 and I can't find a relevant PR (and don't want to spend more time on this kind of actions). I'm opening as a draft before I forgot I started something, and waiting for a fix for qgis/QGIS#63761.

[uclaros]

For reference, here's the PR for the gui part of globe: qgis/QGIS#61220

[DelazJ]

Thank you @uclaros. I've seen that one, and didn't pick it because by "relevant" I meant a PR in which there's enough material we can (easily/directly) use for the docs.
I wrote the description based on various discussion and commits, but I'm still not sure for the globe part, and would be happy to get some more description for the map side. The idea is to better highlight the difference between the two, when would I use one or the other... Your inputs would be more than welcome. Thanks.

[uclaros]

That's a common problem with big features spanning several PRs...
I'll try to add some info here about the differences, but I'm not sure how detailed we want the docs to be, I'll leave it for you to decide :)

3D Map Views

Best choice for Flat Earthers 😁
The 3D view is rendered on a flat plane using the project's CRS if it is projected, or using EPSG:3847 if not.
A DEM/mesh can be used to represent the terrain, however the zero elevation plane is still a flat rectangle covering the project's extent.
The actual extent of the view can be changed, so one can have a 3d view of a single city for example.
2d layers (raster, vector) are rendered on the terrain's surface as a texture
Vector layers can also be rendered as 3d entities if they are 3d, or as extruded polygons if they are 2d.
3d layers (point clouds, 3d tiles) are rendered in 3d
Best for smaller scenes, but effort is being made to handle world wide scenes too.

Globe views

Flat earthers hate them 😁
A 3d terrain entity is created using the project's ellipsoid, so it looks like a sphere
The extent cannot be limited, always includes the whole celestial body.
No support for dem elevations yet.
2d layers can only be rendered as a texture on the ellipsoid, no 3d vectors or extrusions yet
3d layers are rendered in 3d
Best for world wide data and when the earth's curvature matters

Hope the above helps :)