Add WCS.celestial examples to the docs

Sometimes users will encounter this error:
```
NotImplementedError: Currently only data with a 2-d celestial WCS can be reprojected using flux-conserving algorithm
```
when trying to reproject 2D data with extra dimensions in the WCS. The solution is very easy, just use `wcs.celestial`, i.e., the celestial subset of the WCS, instead of the full 3D or 4D WCS. There's presently no example of this in the reproject docs (at least as far as I could see by searching), so we should add one.

Start by reviewing the existing reproject documentation examples and locate the section covering flux-conserving reprojection. Add an example showing that 2D data with extra WCS dimensions can be handled with wcs.celestial, and confirm the documented example addresses the reported NotImplementedError.

Written by the indexing model from the issue text.