Propagate the refactor in Step 10 MathFunctions through all of the
steps. Use MathFunctions/MathFunctions.cxx instead of Tutorial.cxx
to determine which sqrt library is called. Adds .h files which
correspond to their .cxx files by name.
In commit 80f5d28813 (Tutorial: Update step 2 style, 2022-07-25,
v3.25.0-rc1~226^2) we replaced some uses of `USE_MYMATH` with `MY_MATH`.
Restore the former name for consistency with the rest of the tutorial.
Clarify that the process of making a target optional in the second step
of the tutorial is further steps to the work done before, not
a separate addition to the file.
Do this by ensuring that the paragraph which describes the work done
mentions the placement and use of the previous command.
This is done to avoid duplication of the add_subdirectory call.
CMake Issue: #22663
Extend the change from commit 9e5bbb69ea (Tutorial: Add captions for
code blocks, 2021-06-11) to also add a Sphinx named target to code
blocks. Name each target after the change the code block makes.
