docs: clarify uv install paths

[cosmopolitan033]

Problem

The installation guide shows uv add mellea, but issue #757 notes that command assumes an initialized uv project. New users may need either a from-scratch uv project flow or a direct virtualenv install flow.

Solution

Clarify the install section by distinguishing:

• existing pip environments (pip install mellea)
• existing uv projects (uv add mellea)
• new uv projects (uv init ... then uv add mellea)
• quick virtualenv trials (uv venv then uv pip install mellea)

Closes #757.

Validation

• git diff --check

Confidence

Score: 82/100
Category: docs