[DOCS-11746] Update Getting Started in Datadog, part 1: structure

[evazorro]

What does this PR do? What is the motivation?

The Getting Started in Datadog guide is linked from the landing page of the docs site but has grown a little unruly over time and may not be the most helpful starting place for new users of the app. Harry Xue provided a great summary of the issues here.

This PR addresses some of the structural issues: removing and reordering topics on the page, updating the Learning Center links, and cleaning up the Further Reading section. I also fixed a few things that were being flagged by our Vale linter, but otherwise I didn't touch the content (I will do that in a followup PR).

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

The changes are organized by commit and you might find it easier to review that way.

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/eva/DOCS-11746/getting_started/application/

[domalessi]

Def eager for you to improve some of the copy lol. But all looks good so far with what you focused on here -- link updates, content restructure, Further Reading clean-up.

[evazorro]

Def eager for you to improve some of the copy lol.

@domalessi Fear not!! #32238

Thanks for the review!