docs: clarify uv add requires an existing uv project

[SAY-5]

Pull Request

Issue

Fixes #757

Description

The installation docs show uv add mellea, but uv add only works inside an existing uv project (one created with uv init or already holding a pyproject.toml). Users who just want to try Mellea in a plain virtual environment hit an error. This adds a short note clarifying that requirement and pointing to uv pip install mellea as the no-project alternative, matching the maintainer suggestion in the issue thread.

Testing

• Tests added to the respective file if code was changed
• New code has 100% coverage if code was added
• Ensure existing tests and github automation passes (a maintainer will kick off the github automation when the rest of the PR is populated)

Attribution

• AI coding assistants used

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