feat: build plugin docs as part of package-plugin when docsPath is set

[sunker]

What this PR does / why we need it:

Adds a "Build plugin documentation" step to package-plugin that runs npx @grafana/plugin-docs-cli build when docsPath is set in dist/plugin.json. The step generates dist/docs/manifest.json and copies the markdown and images, so multi-page docs ride along in the resulting ZIP and reach the CDN through the existing pipeline.

Plugins without docsPath are unaffected - the bash conditional short-circuits before invoking the CLI.

Which issue(s) this PR fixes:

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

Special notes for your reviewer:

[Copilot]

Pull request overview

Adds an optional documentation build step to the package-plugin composite action so that plugins declaring docsPath can ship multi-page docs artifacts inside the packaged ZIP (and onward to the CDN pipeline).

Changes:

• Adds a “Build plugin documentation” step that runs npx @grafana/plugin-docs-cli build when docsPath is set in dist/plugin.json.

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[academo]

I am not 100% sure we can just assume the runner for this action is going to be debian-based. You can move this into a two steps where a nodejs script reads the json and exports the docs_path and the other uses it.

Unless I am mistaken and we are already running apt commands in other parts of this build action

[sunker]

We already use debian-based commands in this action - see for exampel

plugin-actions/package-plugin/action.yml

Line 135 in ed068c2

sudo apt-get install jq