[DOCS] Enhance OpenTelemetry resource attributes documentation with detailed mapping information

[harshithsaiv]

Description

This PR enhances the OpenTelemetry resource attributes documentation to provide clear explanations about how different types of attributes are mapped to Elastic Common Schema (ECS) fields. It expands on the existing documentation to clarify how unmapped attributes are handled, how OpenTelemetry spans map to APM concepts, and how attribute values are conditionally translated.

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

Closes #elastic/apm-server#15509 (comment)

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>

Labels

This PR is for serverless documentation, so it should have the backport-skip label applied as per guidance for serverless docs. It should also have the appropriate review labels (needs-writer-review, needs-dev-review) added.

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💚 CLA has been signed

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Warning

It looks like this PR modifies one or more .asciidoc files. These files are being migrated to Markdown, and any changes merged now will be lost. See the migration guide for details.

[mergify]

This pull request does not have a backport label. Could you fix it @harshithsaiv? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[github-actions]

Warning

It looks like this PR modifies one or more .asciidoc files. These files are being migrated to Markdown, and any changes merged now will be lost. See the migration guide for details.

[github-actions]

Warning

It looks like this PR modifies one or more .asciidoc files. These files are being migrated to Markdown, and any changes merged now will be lost. See the migration guide for details.