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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[planetf1]

The fix is correct and this is mergeable as a narrowband patch for the immediate confusion.

One note: this leaves the page with three code blocks plus a prose note, which is slightly awkward. The fuller fix — replacing the parallel blocks with headed sections that make the uv project vs venv decision explicit — is tracked in #757 with concrete implementation notes. Up to you whether to extend this PR or merge as-is and let someone pick up #757 separately.

[serjikibm]

Looks like a good narrow fix to the docs just to avoid confusion.