Update markdown documentation

[postscript-dev]

1. Fix formatting of headers in markdown
2. Add policy of reusing test data files (suggested by @piponazo in Fix exiv2: verbose extract stdout mutli-file #2068 (review))

[codecov]

Codecov Report

Merging #2076 (09f566d) into main (46c3290) will decrease coverage by 0.21%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #2076      +/-   ##
==========================================
- Coverage   62.05%   61.83%   -0.22%     
==========================================
  Files          96       96              
  Lines       19197    19103      -94     
  Branches     9835     9819      -16     
==========================================
- Hits        11912    11813      -99     
- Misses       4978     4995      +17     
+ Partials     2307     2295      -12     

Impacted Files	Coverage Δ
src/safe_op.hpp	69.23% <0.00%> (-27.65%)	⬇️
include/exiv2/slice.hpp	69.64% <0.00%> (-21.89%)	⬇️
include/exiv2/error.hpp	60.71% <0.00%> (-4.81%)	⬇️
src/utils.cpp	38.46% <0.00%> (-1.93%)	⬇️
include/exiv2/value.hpp	82.96% <0.00%> (-0.55%)	⬇️
src/enforce.hpp	75.00% <0.00%> (+2.77%)	⬆️
src/getopt.cpp	67.30% <0.00%> (+5.76%)	⬆️
include/exiv2/metadatum.hpp	88.88% <0.00%> (+16.16%)	⬆️

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[piponazo]

LGTM, Thanks!