Add a walkthrough of an actual graph example to explain content

[estherk15]

What does this PR do? What is the motivation?

• Adds an example through graphs of what users are seeing
• This supplements the existing content of the guide
• Editorial review: https://datadoghq.atlassian.net/browse/DOCS-11272

Merge instructions

Merge readiness:

• Ready for merge

[github-actions]

✅ Documentation Team Review

The documentation team has approved this pull request. Thank you for your contribution!

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/esther/docs-update-rollup-guide/dashboards/functions/rollup
• https://docs-staging.datadoghq.com/esther/docs-update-rollup-guide/dashboards/guide/rollup-cardinality-visualizations

[cswatt]

Sorry for this annoying review! I started out understanding it and began to get confused—I think I need to see an equation for the computations here.

[cswatt]

Forgot this note about the graphics. Let's clone/edit the notebook and then take new screenshots:

• it's really not obvious what each colored line represents, and the displayed key is useless. Let's add aliases to the lines, so each graphic has an informative key that says the blue line is 5-min rollup, the purple line is 30-min rollup, etc.
• These are not fractions—they're percentages! Let's replace "Fraction" in each graph title with "Percentage"
• For the last two graphs, it's kind of hard to tell the color difference between the blues. I know it's the classic color palette, but it doesn't make much sense for two groups of two lines, where we're trying to emphasize a difference in mobile vs. total. Maybe we can keep the blue/purple for mobile, and then do a red/yellow for total.

[edanaher]

Overall, I think this looks pretty good, but I had a few questions/comments.