docs(examples): fix multiple example notebooks

[wpbonelli]

Close #2808 and fix some other example notebook failures

• grid_intersection_example.py: removed leading # %% before the YAML front-matter, which was causing jupytext to fail to parse and consequently fail to auto-detect a kernel
• feat_working_stack_examples.py: change df_flux.groupby(lambda x: x.split("_")[-1], axis=1) → df_flux.T.groupby(lambda x: x.split("_")[-1]), since pandas 3.0 removed the axis parameter from DataFrame.groupby()
• plot_map_view_example.py and plot_cross_section_example.py: these used to use PyCharm-style cell markers until chore(dependencies): update conda environment #2414, but that PR missed a few # + lines that now have no matching closing markers. Jupytext's light-format parser swallows everything after these into one giant cell until it processes some markdown further down that superficially looks like another # + marker, which messes up the notebook's cell rendering.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.3%. Comparing base (556c088) to head (3080b2e).
⚠️ Report is 204 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2811      +/-   ##
===========================================
+ Coverage     55.5%    73.3%   +17.7%     
===========================================
  Files          644      659      +15     
  Lines       124135   132182    +8047     
===========================================
+ Hits         68947    96908   +27961     
+ Misses       55188    35274   -19914     

see 585 files with indirect coverage changes

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