Update ClimaCore experiment docs

[juliasloan25]

We have two simple examples in the experiments/ClimaCore/ directory. As long as we are keeping them around, we should keep them well-documented and visible from the docs. They are currently only documented via the README in that directory, which could be more detailed, and comments in the drivers.

To Do

• Update the README in experiments/ClimaCore/
• Add a docs page for the heat diffusion example
• Add a docs page for the sea breeze example