Add browser-use rules documentation (#1732)

Introduced a new markdown file `browser-use-rules.mdc` containing
comprehensive guidelines for contributing to the `browser-use` project.
The document outlines general contribution guidelines, development
rules, and instructions for adding new actions and creating agents. Key
points include the use of structured metadata for AI, type-safe coding
practices with Pydantic, and the preferred method for dependency
installation using `uv`.
    
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## Summary by cubic
Added a new markdown file with clear contribution guidelines for the
browser-use project, including rules for development, adding actions,
and creating agents.

- **Documentation**
- Explains file docstring standards, use of structured metadata, and
type-safe coding with Pydantic.
- Details how to install dependencies with uv and how to define new
actions and agents.

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