Enhance README with service_name details

[jun-mercury]

Description

Added service_name configuration for GitHub actions and clarified precedence for setting service.name.

Link to tracking issue

Fixes #44072

Documentation

The docs are:

• Explicitly stating what service.name represents per the OTel Resource semantic conventions.
• Recommending an explicit service_name (e.g., github-actions) for CI platforms.
• Positioning the repo-derived default as a convenience for small setups—not as a best practice for larger organizations.
• Showing how to use repository Custom Properties only when distinct repos truly map to distinct services.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: codeboten / name: Alex Boten (9a304b2)

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[github-actions]

Welcome, contributor! Thank you for your contribution to opentelemetry-collector-contrib.

Important reminders:

• Please review our Contributing Guidelines.
• Don't forget to sign the Contributor License Agreement (CLA) if you haven't already.

A maintainer will review your pull request soon. Thank you for helping make OpenTelemetry better!

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