gh-41027: Avoid excessive parallelism for doc/bootstrap
    
fix #38753

there's also `docker/setup-make-parallelity.sh` and `src/bin/sage-num-
threads.py` which we should probably prefer to use instead. If you
invoke `meson compile doc-html` with say `-j3`, this isn't respected
either.

But at least this is an improvement of the status quo (99).


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### ⌛ Dependencies

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URL: #41027
Reported by: user202729
Reviewer(s):

Author: Release Manager

src/doc/bootstrap

@@ -183,4 +183,4 @@ cat <<EOF
 
 EOF
 ) > "$OUTPUT_DIR"/index_alph.rst
-sage-package list --has-file SPKG.rst | OUTPUT_DIR=$OUTPUT_DIR OUTPUT_RST=1 xargs -P 99 -n 1 sage-spkg-info
+sage-package list --has-file SPKG.rst | OUTPUT_DIR=$OUTPUT_DIR OUTPUT_RST=1 xargs -P $(nproc 2> /dev/null || echo 8) -n 1 sage-spkg-info