[apm] Apply APM IA changes to serverless docs

[colleenmcginnis]

Description

Applies the updated APM information architecture introduced in #4230 to serverless docs.

Notable differences

• Get started
  • Stateful: There are two methods to choose from when getting started.
  • Serverless: Only one method is applicable to serverless.
• Learn about data types:
  • Stateful: In #4230 we created a page for each data type that folded in content that is related to the data types but was somewhere else in the IA (for example: Distributed tracing is now a part of Traces and Span compression is a part of Spans).
  • Serverless: We didn't copy over the general info on data types from stateful to serverless when initially writing serverless docs. As a result, we can't fold content like Distributed tracing and Span compression into a general page for each data type. Instead, they just exist as their own pages under Learn about data types.
    • 📝 Note: The general data type docs are good candidates for copying over to serverless docs if we don't consolidate serverless/stateful soon, but I'm not doing it here since it would require a technical review.
• Collect application data:
  • Stateful: A few topics that apply to all APM agents were added under APM agents.
  • Serverless: The subpages under APM agents are not applicable to serverless and the Kubernetes and Jaeger pages were never copied over to serverless docs.
    • 📝 Note: The new content on the stateful Collect application data landing page is a good candidate for copying over to serverless docs if we don't consolidate serverless/stateful soon, but I'm not doing it here since it would require a technical review.
• Use APM securely:
  • Stateful: This pulled together all security-related topics across APM agents, APM Server, and Kibana/Applications UI.
  • Serverless: There is only one page of content in the serverless docs.
    • 📝 Note: There are probably other parts of the stateful docs that could be applicable to serverless, but I'm not figuring that out here since it would require a technical review.
• Manage storage / Reduce storage:
  • Stateful: Reduce storage is a page within the Manage storage section.
  • Serverless: Reduce storage is the only page from the stateful Manage storage section that exists in serverless docs so I just used Reduce storage in the nav.
• APM APIs / Managed intake service event API:
  • Stateful: There are two kinds of APIs (Kibana + APM Server).
  • Serverless: There's only one API so I used Managed intake service event API in the nav instead of APM APIs.
• Troubleshooting:
  • Stateful: There's a lot more content because of APM Server troubleshooting.
  • Serverless: There's only one page of troubleshooting content.
• These sections from the stateful docs are not included in existing serverless docs:
  • Configure APM Server (not applicable to serverless)
  • Monitor APM Server (not applicable to serverless)
  • Upgrade (not applicable to serverless)
  • Release notes (not applicable to serverless)
  • Known issues (maybe applicable? consider adding in the future?)

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

N/A

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

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[mergify]

This pull request is now in conflict. Could you fix it @colleenmcginnis? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b serverless-apm-ia upstream/serverless-apm-ia
git merge upstream/main
git push upstream serverless-apm-ia

[dedemorton]

I think these IA changes are a big improvement. Nice work! I just had a couple of comments about stuff that you can defer, if you want, because it's not related to the IA changes.

Here are a few notes about stuff you probably already know about (just including the info here for the sake of completeness).

These topics exist in stateful, but not serverless (probably aren't supported and/or weren't in the original serverless docs):

• https://www.elastic.co/guide/en/observability/current/apm-mobile-service-overview.html
• https://www.elastic.co/guide/en/observability/current/apm-mobile-session-explorer.html
• https://www.elastic.co/guide/en/observability/current/apm-agent-explorer.html

This topic uses short title in stateful, but not in serverless: https://observability-docs_bk_4593.docs-preview.app.elstc.co/guide/en/serverless/current/observability-synthetics-mfa.html.

(So does this topic, but I think it's appropriate given that "project" is an overloaded term: https://observability-docs_bk_4593.docs-preview.app.elstc.co/guide/en/serverless/current/observability-synthetics-configuration.html)

[dedemorton]

LGTM (assuming build passes, of course)

[dedemorton]

LGTM