doc: GUI Python dependency is not documented #33146

1. 
   fanquake commented at 3:31 pm on August 6, 2025: member

   See this thread: #31233 (review)

   (Actually, while testing, a missing python is now a hard error already when -DBUILD_GUI=ON. So I don’t think the wording “refactor” accurately represents the changes here.)

   This bug still exists. rm -rf ./bld-cmake && cmake -B ./bld-cmake -DBUILD_GUI=ON now fails with:

   So it would be good to revert this pull, or adjust the docs:

   We need to decide whether to document the GUI dep, or re-adjust the code. cc @hebasto

2. fanquake added the label Docs on Aug 6, 2025

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