What does this PR do?

From https://github.com/elastic/observability-docs/issues/1019:

Let's audit the documentation for our top 5 integrations. The descriptions can be improved so users have a better understanding of what benefit they offer, how Elastic can help with the user's use case, and persuade them to install the integration.

This PR adds more context the Linux Metrics integration including:

• Applies a new proposed structure to be applied across top integrations (overview, data types, requirements/prerequisites, setup, reference).
• Creates a soft division between guide-like content and reference-like content.
• Adds context to the "Overview":
  • Acknowledges the relationship between this integration and the larger ecosystem of Elastic products.
  • Provides [placeholder text for] a basic example to illustrate how this integration may be useful.
• Adds a "Data types" section:
  • Provides a high-level overview of the kind of data that is collected. (It's difficult to derive a summary from just the reference sections since they're so long.)
  • Reinforces what "Metrics" are in the context of the integration.
• Adds a "Requirements" section:
  • Reinforces how this integration fits into the larger Elastic system.
  • Mentions trouble that might occur related to permission levels.
• Points the reader to the Getting started guide for step-by-step "Setup" instructions.

For the reviewer

• I had trouble coming up with an example. Can anyone help me fill in the blanks in the third paragraph?
• In some places this is referred to as the "Linux integration" and in others it's referred to as the "Linux Metrics integration". Which is preferred?
• I noticed we're using some terms inconsistently. Here we use the term "data stream" to describe the various metrics, but in the Windows ([docs] Add context to Windows integration documentation #3352) and System ([docs] Add context to System integration documentation #3306) integration docs we refer to them as "datasets". Do these two terms actually mean the same thing or should they be different? If they mean the same thing, which is preferred?
• Open to any and all feedback on language and truthiness!

Checklist

• I have reviewed tips for building integrations and this pull request is aligned with them.
• I have verified that all data streams collect metrics or logs.
• I have added an entry to my package's changelog.yml file.
• I have verified that Kibana version constraints are current according to guidelines.

Author's Checklist

• Review by @bmorelli25
• Review by TBD (Integrations team)

Related issues

• https://github.com/elastic/observability-docs/issues/1019