Add Example Usage of Compute Utils to docs

[dpanici]

trafficstars

Would be useful to show how to use surface_averages etc in docs for users, as it is easier than them looking at the source and otherwise hey wouldn't even know these utilities exist

[unalmis]

Maybe those utility methods should be added to the public api?

Note to self: copy/move the last chunk of this section https://desc-docs.readthedocs.io/en/stable/notebooks/dev_guide/grid.html#A-common-operation-which-relies-on-node-area:-surface-integrals to a dev guide on using utility functions in #353

[dpanici]

make an integrals subfolder in desc