docs: add Installation page (uv install path)

[AllAboutDocs]

Summary

Adds a new docs/INSTALLATION.md page covering prerequisites, installing uv, and installing MkDocs 2.0's pre-release with it. Based on hands-on testing of the full install flow.

What's included

• Python 3.10+ stated as MkDocs 2.0's current prerequisite, framed around uv fetching a suitable interpreter automatically rather than assuming the reader has one.
• Checking for / installing uv via the standalone installer, including the source $HOME/.local/bin/env step needed when uv isn't recognized right after install.
• Installing MkDocs via uv tool install mkdocs --prerelease allow, with an explicit note that this differs from pip's --pre flag — passing --pre to uv doesn't error, it silently installs the latest stable release instead (confirmed firsthand).
• A troubleshooting path for uv's other silent-fallback case: no Python ≥3.10 interpreter available to it, walked through via uv python list → uv python install → reinstalling with --python explicitly.
• Verifying the install via uv tool list, since mkdocs --version no longer exists in 2.0.dev4.

Intentionally not included yet

• Windows steps. I have a Windows machine and will test uv's install flow there separately — this page currently documents macOS/Linux only.
• The pip path from issue Installation docs... #57. Going with uv only for now per your answer there; can add a concise pip/venv section later if we still want both documented.

Testing

Confirmed via mkdocs serve that the page renders correctly at /installation/, and reproduced each command/flag behavior described here firsthand (starting from a stock macOS install with only Python 3.9.6 via Xcode CLT present, and uv installed fresh).