Add contribution guide and clarify local development workflow

[maximepvrt]

Currently, there is no dedicated contribution page to guide new developers through the setup and plugin creation process.

While the AGENTS.md file provides some context, it lacks critical information regarding the local development environment. Specifically, it does not mention that the monorepo requires running package compilation pnpm run dev:packages in parallel with the vue playground pnpm run dev.

Additionally, when running pnpm run dev:playground, the application currently displays the default Nuxt starter page.

[hiSandog]

For this UI bug, a small regression test should probably assert the visible state rather than only the underlying data. A screenshot, DOM assertion, or component test around AGENTS.md would make sure the fix covers what users actually see, especially if the data is already present but rendered incorrectly.