Add missing demo data directory entry to config documentation

[djhoese]

This was brought up by @gerritholl on slack. This specifies the default value of '.' and documents it including a linking to the demo data module.

• Closes #xxxx
• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #1765 (ae97c0e) into main (4d34625) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1765      +/-   ##
==========================================
+ Coverage   92.82%   92.84%   +0.02%     
==========================================
  Files         263      263              
  Lines       38717    38717              
==========================================
+ Hits        35940    35948       +8     
+ Misses       2777     2769       -8     

Flag	Coverage Δ
behaviourtests	4.80% <ø> (+<0.01%)	⬆️
unittests	93.38% <ø> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
satpy/_config.py	98.59% <ø> (ø)
satpy/tests/reader_tests/test_vii_base_nc.py	98.78% <0.00%> (+1.21%)	⬆️
satpy/tests/reader_tests/test_nwcsaf_msg.py	98.31% <0.00%> (+1.68%)	⬆️
satpy/tests/reader_tests/test_vii_l1b_nc.py	100.00% <0.00%> (+2.81%)	⬆️
satpy/tests/reader_tests/test_vii_l2_nc.py	100.00% <0.00%> (+6.06%)	⬆️

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[coveralls]

Coverage remained the same at 93.333% when pulling ae97c0e on djhoese:bugfix-config-demo-data-dir into 4d34625 on pytroll:main.