[Rules] Document pre-deployment validation

[mbullock1986]

Summary

• Document automatic Rules dashboard validation before deployment.
• Document Rulesets API validation with the dry_run=true query parameter.
• Add a Rules changelog entry with an API example.

Related: SHIP-11749

Documentation checklist

• Is there a changelog entry (guidelines)?
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page - an issue has been opened in relation to any incorrect or out of date information that this PR fixes.

[cloudflare-docs-bot]

Review

✅ No issues found in commit eeabdc0.

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