docs: document raising ulimit and vm.max_map_count for QuestDB in Docker

[PedroHenrique0713]

What

Adds a "Raise system limits" section to the Docker deployment guide, documenting how to raise the two per-process OS limits QuestDB needs when running in a container: open files (ulimit -n) and memory-mapped areas (vm.max_map_count).

Why

On Docker, host ulimit settings do not propagate into the container, so a fresh container can log the warning reported in #370:

fs.file-max limit is too low [current=524288, recommended=1048576]

The Docker guide did not cover how to fix this, and the capacity planning guide only documents the systemd / limits.conf path, not the container one. This section fills that gap:

• Open files: docker run --ulimit nofile=1048576:1048576, the Docker Compose ulimits equivalent, and default-ulimits in /etc/docker/daemon.json as a global alternative.
• vm.max_map_count: set on the host via sysctl, since it is a host kernel setting shared with the container. This is the sibling limit QuestDB checks at startup, so I documented both together to match the startup check.

All values use the officially recommended 1048576 from the capacity planning guide, which the new section links to.

Closes #370