Add experimental lifecycle to changelog tooling

[lcawl]

Relates to #3310

This PR adds the experimental lifecycle value for use in the changelog tooling.
Some tests needed to be fixed to accommodate the change.

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📥 Commits

Reviewing files that changed from the base of the PR and between 477a319 and 338d942.

📒 Files selected for processing (6)

• config/changelog.example.yml
• docs/contribute/configure-changelogs-ref.md
• src/Elastic.Documentation.Configuration/Changelog/ChangelogConfiguration.cs
• src/Elastic.Documentation/Lifecycle.cs
• tests/Elastic.Changelog.Tests/Changelogs/ChangelogConfigurationTests.cs
• tests/Elastic.Markdown.Tests/Directives/ChangelogConfigTests.cs

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📝 Walkthrough

Walkthrough

This PR adds Experimental as a new lifecycle value to the changelog system. The change includes: defining the Experimental enum member in Lifecycle.cs with proper display attributes and documentation, updating the default lifecycle configuration to include it, documenting the new value in the reference guide and example configuration, and updating tests to verify the new lifecycle is included in defaults and properly loaded from configuration files.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: adding the experimental lifecycle value to the changelog tooling.
Description check	✅ Passed	The description clearly relates to the changeset, explaining that it adds the experimental lifecycle value and notes that tests were updated accordingly.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

✨ Simplify code

• Create PR with simplified code
• Commit simplified code in branch tech-preview-lifecycle

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