Clarify Scene Documentation without Readers

[joleenf]

Updates the scene documentation to clarify that an xarray.DataArray can be added manually without a reader.

• Closes Confusing documentation for creating a Scene without a filename or reader #2836

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.97%. Comparing base (fc95ee1) to head (f3a2343).
Report is 530 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2868      +/-   ##
==========================================
+ Coverage   95.94%   95.97%   +0.02%     
==========================================
  Files         366      368       +2     
  Lines       53584    53973     +389     
==========================================
+ Hits        51412    51801     +389     
  Misses       2172     2172              

Flag	Coverage Δ
behaviourtests	4.02% <ø> (-0.03%)	⬇️
unittests	96.07% <ø> (+0.02%)	⬆️

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[coveralls]

Pull Request Test Coverage Report for Build 10220258210

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.073%

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Totals
Change from base Build 10140739920:	0.0%
Covered Lines:	52030
Relevant Lines:	54157

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💛 - Coveralls