Add hotpatch HLD document

Add the high-level design document for the SONiC hotpatch feature.

The hotpatch feature enables applying software patches to a running
SONiC system without requiring a full image upgrade or reboot.

Document covers:
- Hierarchical patch packaging model (Hotfix Package -> Sub-patches -> Packages)
- Patch metadata format (patch_info.yml, summary.yml)
- Six patch types: docker, debian, script, hook_script, func_hotpatch, dockerfile
- libcareplus internals for function-level live patching
- Build pipeline tooling (generate.py, check.py, build.sh, package.py)
- Installation flow and ordered rollback mechanism
- sonic_installer CLI enhancements for patch management
- Safety checks, restrictions, and test requirements

Signed-off-by: yuezhou.jk <yuezhou.jk@alibaba-inc.com>

Author: yuezhoujk