docs: add Podman installation documentation (#8646)

* docs: add Podman installation documentation

- Add new podman.md with comprehensive installation and usage guide
- Cover installation on multiple platforms (Ubuntu, Fedora, Arch, macOS, Windows)
- Document GPU support (NVIDIA CUDA, AMD ROCm, Intel, Vulkan)
- Include rootless container configuration
- Document Docker Compose with podman-compose
- Add troubleshooting section for common issues
- Link to Podman documentation in installation index
- Update image references to use Docker Hub and link to docker docs
- Change YAML heredoc to EOF in compose.yaml example
- Add curly brackets to notice shortcode and fix link

Closes #8645

Signed-off-by: localai-bot <localai-bot@users.noreply.github.com>

* docs: merge Docker and Podman docs into unified Containers guide

Following the review comment, we have merged the Docker and Podman
documentation into a single 'Containers' page that covers both container
engines. The Docker and Podman pages now redirect to this unified guide.

Changes:
- Added new docs/content/installation/containers.md with combined Docker/Podman guide
- Updated docs/content/installation/docker.md to redirect to containers
- Updated docs/content/installation/podman.md to redirect to containers
- Updated docs/content/installation/_index.en.md to link to containers

Signed-off-by: LocalAI [bot] <localai-bot@users.noreply.github.com>
Signed-off-by: localai-bot <localai-bot@users.noreply.github.com>

* docs: remove podman.md as docs are merged into containers.md

Signed-off-by: localai-bot <localai-bot@users.noreply.github.com>

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Signed-off-by: localai-bot <localai-bot@users.noreply.github.com>
Signed-off-by: LocalAI [bot] <localai-bot@users.noreply.github.com>
Co-authored-by: localai-bot <localai-bot@users.noreply.github.com>