Add section to let CMake manage the Kokkos dependency

[pzehner]

Add a section to integrate Kokkos with CMake FetchContent. The hybrid external/internal dependency code cannot be displayed due to insufficient space on the pages. Not a perfect solution, but it's aligned with the official documentation.

Preview:

[tpadioleau]

In the end I don't think we should go into the details of how to use FetchContent etc. This is largely opinionated. I would rather describe the package name, the targets and the components available. Similar to the Targets tab in https://conan.io/center/recipes/ginkgo. We can also mention that Kokkos supports being added as a subdirectory, this implies for example that Kokkos::kokkos also exists which is not obvious.

[pzehner]

I don't know, I think it gives practical directions about "how to include Kokkos as a dependency to my project," which is unfortunately not trivial at this time.

You cannot simply do:

conan install kokkos

[pzehner]

I could restore the page layout, but I cannot show the external/internal dependency approaches on the page, as it would take too much space. I just added a comment saying that it is possible. I think it's a start nevertheless.