build/pkgs/*/SPKG.rst: Explain how SageMath makes use of the package

[mkoeppe]

From #37581 (comment)

I'd like to expand some of the package descriptions in the SPKG.rst with a description of how the package is being used in the Sage project. (That's part of a larger effort to improve the relation of Sage with upstream projects.)

[kwankyu]

We may start by writing something about this direction in the developer guide. Then whenever a package is updated, the description may be improved.

[mkoeppe]

Good idea!