Add docs link verification task

[alvarosanchez]

Summary

• Add a checkDocsLinks Gradle task for Micronaut docs projects.
• Validate local file and fragment links in generated index.html and configurationreference.html.
• Keep docs unchanged so CI can invoke link checking explicitly.

Verification

• ./gradlew :micronaut-gradle-plugins:test --tests io.micronaut.build.docs.ValidateGuideLinksTaskSpec
• ./gradlew :micronaut-gradle-plugins:functionalTest --tests io.micronaut.build.docs.DocsLinksFunctionalTest
• git diff --check origin/HEAD...HEAD

Closes #181

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[alvarosanchez]

@sdelamo did you want to validate external links when you created #181? Because this PR is not checking that