reducing test times for DGM

[ayushinav]

Following from the last comment in #802, this reduces the run times for dgm tests.
@sathvikbhagavan @ChrisRackauckas

[sathvikbhagavan]

Left a few small formating changes in the tutorial.
One more thing is the tutorial won't run in the doc build. Use @example ... syntax instead of julia markdown. Ref - https://documenter.juliadocs.org/stable/man/syntax/#@example-block

[ayushinav]

Left a few small formating changes in the tutorial. One more thing is the tutorial won't run in the doc build. Use @example ... syntax instead of julia markdown. Ref - https://documenter.juliadocs.org/stable/man/syntax/#@example-block

Thanks, made the changes. I was curious why the plot did not render as such in the dev docs. I looked at gpu.md which used julia instead of @example but ig it's the only that does so.