Adding docs for AMReX-Kokkos build

[nataraj2]

Summary

This PR adds the docs for building a code that uses both AMReX and Kokkos. An example is given in amrex-devtests/kokkos.

Additional background

The instructions for building the AMReX-Kokkos example are general, but specific instructions for building on Perlmutter are given.

Checklist

The proposed changes:

• fix a bug or incorrect behavior in AMReX
• add new capabilities to AMReX
• changes answers in the test suite to more than roundoff level
• are likely to significantly affect the results of downstream AMReX users
• include documentation in the code and/or rst files, if appropriate

[WeiqunZhang]

We don't have any instructions on building AMReX on specific machines (e.g., perlmutter), because they are likely to be outdated at some point. So I don't think we need to have instructions on how to build AMReX and Kokkos, especially because Kokkos users are probably already used to cmake. (If not, they can ask ChatGPT for help on cmake.)

[nataraj2]

Sounds good. Closing this PR.