[Chapel Merge] Update prereqs for docs build changes

Branch: refs/heads/main
Revision: fc02a6e
Author: mppf
Log Message:

Merge pull request #18539 from mppf/update-prereqs

Update prereqs for docs build changes

Resolves #18536
Resolves #18537

  • add a section for optional prereqs
  • focus on what is required and then the reason (easier to explain when
    multiple elements need the same thing)
  • note cmake & doxygen required for full docs build

Future Work: I'm not certain that we need python-devel anymore for
make docs to work. It was added in #2254 and I don't know why.

Reviewed by @lydia-duncan - thanks!

Modified Files:
M doc/rst/usingchapel/prereqs.rst

Compare: https://github.com/chapel-lang/chapel/compare/ff67b8863883...fc02a6e6d695