Point out get_area_def in resample documentation

[gerritholl]

In the main resample documentation, mention the nice satpy utility
function get_area_def.

• Closes Point out get_area_def in resample documentation #1726
• Fully documented

[codecov]

Codecov Report

Merging #1726 (efe4b60) into main (8a85d3b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1726   +/-   ##
=======================================
  Coverage   92.58%   92.58%           
=======================================
  Files         258      258           
  Lines       38345    38345           
=======================================
  Hits        35501    35501           
  Misses       2844     2844           

Flag	Coverage Δ
behaviourtests	4.83% <ø> (ø)
unittests	93.12% <ø> (ø)

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Impacted Files	Coverage Δ
satpy/resample.py	80.24% <ø> (ø)

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[coveralls]

Coverage remained the same at 93.072% when pulling efe4b60 on gerritholl:doc-refer-to-get-area-def into 8a85d3b on pytroll:main.

[djhoese]

My excuse for this is that this function wasn't supposed to be public originally, but it seems it has become more and more useful and needed. Thanks for this update.