gh-40696: Add one "long time" annotation
    
The main goal here was to add a `# long time` to the first test in
`src/sage/tests/books/computational-mathematics-with-
sagemath/integration_doctest.py`, since it's causing "slow doctest"
warnings in the CI.

After that, the header (which promises that the file can be deleted and
re-generated) needed to be updated.

Finally, I made the indent (now four spaces, was two) consistent with
the rest of sage. Yes this would make a diff harder to read; no I don't
think anyone is ever going to update this file from the TeX source ever
again.
    
URL: #40696
Reported by: Michael Orlitzky
Reviewer(s):

Author: Release Manager