Fix documentation workflow and include standards_export extension in docs

[AlexDo1]

This PR closes #293.

[AlexDo1]

@mmaelicke

Suddenly, 'otuside' workflows are not allowed anymore:

This setting can be modified here but I wonder where this change is coming from.

[codecov[bot]]

Codecov Report

Merging #294 (328d661) into main (a5cdf2e) will not change coverage. The diff coverage is 88.88%.

@@           Coverage Diff           @@
##             main     #294   +/-   ##
=======================================
  Coverage   64.20%   64.20%           
=======================================
  Files          75       75           
  Lines        3878     3878           
=======================================
  Hits         2490     2490           
  Misses       1388     1388           

Flag	Coverage Δ
e2e	64.20% <88.88%> (ø)

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

Impacted Files	Coverage Δ
metacatalog/ext/standards_export/extension.py	89.90% <ø> (ø)
metacatalog/models/entry.py	68.62% <87.50%> (ø)
metacatalog/__version__.py	100.00% <100.00%> (ø)

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[AlexDo1]

The Documentation Github CI works again.
I had to add the installation of additional dependencies for the standards_export extension (jinja2, lxml) to the Github action.

[AlexDo1]

@mmaelicke StandardsExportExtension is still not shown in the docs.