Document Agent Skills Marketplace in maui-labs

## Context

dotnet/maui-labs PR [#58](https://github.com/dotnet/maui-labs/pull/58) added a **plugin marketplace** for distributing agent skills compatible with Copilot CLI, Claude Code, and VS Code.

The `pr-docs-check` workflow attempted to create a draft PR on this repo but lacked cross-repo permissions.

## What needs documenting

- The `plugins/` directory in maui-labs hosts installable agent skills
- Installation command: `/plugin marketplace add dotnet/maui-labs` → `/plugin install dotnet-maui@dotnet-maui-labs`
- Skills cover DevFlow automation, profiling, accessibility, platform bindings, and diagnostics
- Contribution guide: [plugins/CONTRIBUTING.md](https://github.com/dotnet/maui-labs/blob/main/plugins/CONTRIBUTING.md)

## Reference

- PR: https://github.com/dotnet/maui-labs/pull/58
- Marketplace config: [`.github/plugin/marketplace.json`](https://github.com/dotnet/maui-labs/blob/main/.github/plugin/marketplace.json)
- Plugin catalog: [`plugins/README.md`](https://github.com/dotnet/maui-labs/blob/main/plugins/README.md)

Start by reading the linked plugins/README.md, plugins/CONTRIBUTING.md, and .github/plugin/marketplace.json in maui-labs, then find the appropriate location in the docs-maui repository. Done means the documentation explains the plugins directory, installation commands, skill areas, and contribution guide clearly.

Written by the indexing model from the issue text.