Update introduction to solutions

[lcawl]

Note

Please note that for this PR, we have omitted use cases and concepts from Elasticsearch while we wait for confirmation from the Platform team about the Elasticsearch story, and how it's used as a solution vs. platform. There are also next steps in our onboarding efforts to revise and align our solution landing pages.

Relates to #2795

This PR updates the Use cases page to describe the differences between the solutions.

[github-actions]

🔍 Preview links for changed docs

• get-started/introduction.md

[benironside]

Great work. This is a really nice overview, very succinct and a good level of detail. Left some comments for your consideration. I hope they are helpful.

[leemthompo]

I have no issues with the wording or anything here, looks good! But I'm concerned about scattering info, based on two main reactions to reading this:

First, I'm not sure why we aren't adding this content to the solutions landing page which is comparably quite sparse, despite being the section landing page? My instinct would be to make the section under fundamentals just a brief page that basically links out to /solutions (i.e. swap the content of get-started/introduction.md and solutions/index.md).

With this addition, the solutions and use cases now have five landing page surfaces that all overlap quite strongly:

• This new section under fundamentals
• The solutions/use cases landing page
  • The Elasticsearch solution landing page
  • The Observability solution landing page
  • The Security solution landing page

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My next question is about the asymmetry of treatment of the 3 solutions/use cases: Observability and Security have detailed dropdowns with big hyperlinked lists. The fact that we've put these in dropdowns gives me the impression that this is too much info for this surface, which reinforces my concerns about scattering info from my first point.

Given ES has the broadest set of use cases and features, it's not super clear to me why it's given comparatively briefer treatment here, with no dropdowns, for example.

I don't consider any of this a giant problem, just my impression of the changes and perhaps there are follow-up plans I'm not aware of. I might be overthinking this :)