Add doc to release workflow

[kevinbackhouse]

We weren't running the doxygen step in the "release" workflow.
I have tested this on my fork: https://github.com/kevinbackhouse/exiv2/actions/runs/1110388655

[hassec]

LGTM! 👍

Future goal should probably be to somehow directly host the doc somehow so one could have a look at it... But that's a problem for the future :D

[codecov]

Codecov Report

Merging #1861 (2da1bcb) into 0.27-maintenance (e649363) will not change coverage.
The diff coverage is n/a.

@@                Coverage Diff                @@
##           0.27-maintenance    #1861   +/-   ##
=================================================
  Coverage             46.21%   46.21%           
=================================================
  Files                   146      146           
  Lines                 22874    22874           
  Branches              11762    11762           
=================================================
  Hits                  10572    10572           
  Misses                 6696     6696           
  Partials               5606     5606           

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