Add annotations documentation

[evazorro]

What does this PR do? What is the motivation?

Adds a guide to the new annotations feature for timeseries widgets, including creation, bulk creation, editing, and deletion.

Merge instructions

I'd like to add a screenshot before merge (see comment inline) done

Merge readiness:

• Ready for merge

Additional notes

[evazorro]

I'd like to add a screenshot -- @amymzhou, what would be a few good example metrics to show?

[amymzhou]

Here's a Org 2 link you can use! https://app.datadoghq.com/notebook/12907421/annotations-demo

The data is "real", but the chart title and annotation text is fake, so it's ok to show on our public docs site

[amymzhou]

It's currently in a notebook, but you can copy it to a dashboard too if you prefer a screenshot of a dashboard widget

[github-actions]

Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/eva/DOCS-12263/dashboards/annotations/

[amymzhou]

@evazorro I feel like Step 1 is misleading, since people would create an annotation (Steps 1-4 in adding an annotation), then use the dropdown to select if they want to add it to more than one widget. Because of that, I feel like we can remove Step 1 "Select the widgets you want to..." altogether.

But maybe we can expand a bit more on Step 3 "Choose All widgets or selected widgets"

[evazorro]

Definitely happy to remove that step! I was trying to account for the "apply to selected widgets" case assuming that you'd need to select the widgets ahead of time. But now that I'm trying it again, I see that you can check/uncheck widgets in the list after choosing that option. I'll revise!

[amymzhou]

LGTM