docs: add Core-execution-flow page with architecture flowcharts

New 'Core-execution-flow' page provides a comprehensive deep-dive into
GeneralUpdate.Core's execution architecture, covering:

- Three-layer dispatch design (Bootstrap → RoleStrategy → OSStrategy)
- Complete ClientStrategy workflow with flowcharts
- DownloadPlanBuilder Chain vs Full decision logic (80% threshold)
- Middleware pipeline (Hash → Compress → Patch) execution differences
- DiffPipeline differential engine internals (CleanAsync/DirtyAsync)
- Chain-to-Full fallback mechanism with version tracking
- IPC communication protocol (AES-256-CBC encrypted)
- UpdateStrategy Upgrade-process execution flow
- Silent Mode delayed-upgrade design
- OS strategy platform differences
- Error recovery panorama (13 error scenarios)
- Key code path index with source links

Placed in Components sidebar, directly after GeneralUpdate.Core.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: JusterZhu