docs: add file descriptor limits section to Docker guide

[raj00700777-ctrl]

Adds a "File descriptor limits" section to the Docker deployment guide, addressing #370.

• Docker's default per-container ulimits are often too low for QuestDB, which opens one file handle per column per partition; the docs didn't previously mention this
• Explains the fs.file-max limit is too low startup warning and why raising the host's ulimit -n alone doesn't fix it inside a container
• Shows both the daemon-wide fix (/etc/docker/daemon.json with default-ulimits) and a per-container fix (docker run --ulimit), so readers can pick based on whether other containers on the host need the higher limit too