Replace doc references to PPP_CONFIG_DIR

[gerritholl]

Replace documentation references to PPP_CONFIG_DIR to refer to
SATPY_CONFIG_PATH instead, except where the context refers to the former
being out of date.

• Closes Replace doc references to PPP_CONFIG_DIR #1724
• Fully documented

[mraspaud]

LGTM, thanks for spotting and replacing these!

[codecov]

Codecov Report

Merging #1724 (c52b8b4) into main (8a85d3b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1724   +/-   ##
=======================================
  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/plugin_base.py	88.00% <ø> (ø)
satpy/resample.py	80.24% <ø> (ø)

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[coveralls]

Coverage remained the same at 93.072% when pulling c52b8b4 on gerritholl:outdated-doc-ppp-config-dir into 8a85d3b on pytroll:main.

[djhoese]

Thanks!