Add Pygac reference to avhrr_l1b_gaclac documentation

[sfinkens]

Add Pygac reference to avhrr_l1b_gaclac documentation.

• Closes avhrr_l1b_gaclac fails to read most files from NOAA CLASS #2494

[codecov]

Codecov Report

Merging #2501 (25f2def) into main (1f97c0b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #2501   +/-   ##
=======================================
  Coverage   94.84%   94.84%           
=======================================
  Files         337      337           
  Lines       49571    49571           
=======================================
  Hits        47016    47016           
  Misses       2555     2555           

Flag	Coverage Δ
behaviourtests	4.41% <ø> (ø)
unittests	95.46% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/readers/avhrr_l1b_gaclac.py	95.67% <ø> (ø)

[coveralls]

Pull Request Test Coverage Report for Build 5187576661

• 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 95.414%

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Totals
Change from base Build 5141473420:	0.0%
Covered Lines:	47129
Relevant Lines:	49394

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💛 - Coveralls

[mraspaud]

Thanks for adding this, very helpful!