Fix stateful vs serverless attribute conflicts

[colleenmcginnis]

⚠️ Do not merge before elastic/docs#3109 merges!

Description

Uses obs-serverless instead of observability in the serverless docs in most cases.

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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[colleenmcginnis]

I initially used master because I thought we wanted to link to the master version of stateful docs, but that messed up code blocks using version (example). 😬

[colleenmcginnis]

For the reviewer: You can see the result of this change by going to this diff in the built-docs repo or looking at diff page and comparing the production link (example) to the preview link (example).

Production	Preview

[bmorelli25]

Clicked around a bunch and LGTM!