Improvements in custom reader documentation

[gerritholl]

In the development guide section on writing a custom reader, point out
where the YAML file goes. Also fix various small markup warts and typoes.

• Fully documented

[codecov]

Codecov Report

Merging #2104 (5940f3a) into main (20434bf) will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2104      +/-   ##
==========================================
+ Coverage   93.89%   93.95%   +0.05%     
==========================================
  Files         283      283              
  Lines       42589    42881     +292     
==========================================
+ Hits        39991    40290     +299     
+ Misses       2598     2591       -7     

Flag	Coverage Δ
behaviourtests	4.77% <ø> (+0.06%)	⬆️
unittests	94.51% <ø> (+0.05%)	⬆️

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

Impacted Files	Coverage Δ
satpy/tests/test_multiscene.py	99.45% <0.00%> (-0.03%)	⬇️
satpy/readers/fci_l2_nc.py	99.45% <0.00%> (-0.01%)	⬇️
satpy/readers/cmsaf_claas2.py	100.00% <0.00%> (ø)
satpy/tests/test_modifiers.py	100.00% <0.00%> (ø)
satpy/tests/test_composites.py	100.00% <0.00%> (ø)
satpy/tests/reader_tests/test_cmsaf_claas.py	100.00% <0.00%> (ø)
satpy/tests/reader_tests/test_seviri_l1b_icare.py	100.00% <0.00%> (ø)
satpy/tests/test_scene.py	99.47% <0.00%> (+0.01%)	⬆️
satpy/readers/fci_l1c_nc.py	98.05% <0.00%> (+0.02%)	⬆️
satpy/tests/test_yaml_reader.py	99.60% <0.00%> (+0.03%)	⬆️
... and 8 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 20434bf...5940f3a. Read the comment docs.

[djhoese]

Very nice. I only have a few small requests.

[coveralls]

Coverage increased (+0.03%) to 94.453% when pulling 5940f3a on gerritholl:custom-reader-doc-improvements into 56eb21b on pytroll:main.