Create Plugin: Add validate-docs workflow to scaffolded plugins

[sunker]

What this PR does / why we need it:

Adds a validate-docs.yml GitHub Actions workflow to the create-plugin scaffolding. The workflow runs @grafana/plugin-docs-cli validate --strict on PRs and pushes that touch docs/** or src/plugin.json, giving plugin authors early feedback on documentation errors before they reach the release pipeline.

The workflow is a no-op for plugins that don't have docsPath set in plugin.json, so all newly scaffolded plugins can safely include it without any impact on existing workflows. The paths: trigger ensures it only runs when docs actually change.

Which issue(s) this PR fixes:

No issue — part of the multi-page plugin docs initiative.

Special notes for your reviewer:

Pair with grafana/plugin-actions#219, which adds the docs build step to the release pipeline.

Assumption: docsPath is conventionally docs. Plugins using a different folder name would need to update the paths: filter in the workflow file.

[github-actions]

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