Generate session-wise executive summaries

[tsalo]

Closes #1200.

Changes proposed in this pull request

• Loop over sessions when generating executive summaries.
• Don't install sphinx-argparse 0.5.0 (copied from fix(doc-build): Address errors/warnings in RTD builds nipreps/fmriprep#3325).

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 83.53%. Comparing base (80d384e) to head (346b20e).

Files	Patch %	Lines
xcp_d/reports/core.py	87.50%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1202      +/-   ##
==========================================
+ Coverage   83.50%   83.53%   +0.02%     
==========================================
  Files          54       54              
  Lines        6894     6892       -2     
  Branches      988      990       +2     
==========================================
  Hits         5757     5757              
+ Misses        874      872       -2     
  Partials      263      263              

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