frontend: Expose Activity API to plugins

[sniok]

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[Copilot]

Pull Request Overview

Expose the Activity API to plugins by exporting Activity, refactoring its Redux slice into a standalone file, and adding documentation plus an example plugin demonstrating usage.

• Extracted activity state management into activitySlice.tsx and exported Activity to plugin consumers.
• Added example plugin (activity) and updated typedoc entrypoints and documentation.
• Updated tests, snapshots, and README indices accordingly.

Reviewed Changes

Copilot reviewed 19 out of 22 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
plugins/headlamp-plugin/src/index.ts	Exports Activity for plugin consumption.
plugins/examples/activity/**	New example plugin demonstrating Activity API usage.
plugins/README.md	Adds link to new example.
frontend/tsconfig.json	Includes Activity component for API docs generation.
frontend/src/redux/reducers/reducers.tsx	Updates import to use new activitySlice reducer.
frontend/src/plugin/index.ts	Exposes Activity via global pluginLib.
frontend/src/plugin/snapshots/pluginLib.snapshot	Snapshot updated to include Activity API.
frontend/src/components/activity/activitySlice.tsx	New Redux slice for activity state management.
frontend/src/components/activity/Activity.tsx	Removes inlined slice; now imports external slice.
frontend/src/components/activity/Activity.test.tsx	Adjusts imports to new slice file.
frontend/src/components/activity/Activity.stories.tsx	Adjusts imports to new slice file.
docs/development/plugins/functionality/index.md	Adds Activity feature documentation.

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[k8s-ci-robot]

PR needs rebase.

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