docs: refresh design documentation

[ZIYAN137]

Summary

• Rework document/ as the formal design documentation entry point.
• Keep fine-grained mdBook navigation while making pages concise and design-focused.
• Move API/function/field-level detail expectations to rustdoc and source comments.
• Add current subsystem pages for architecture, boot, VFAT, shared memory, mutex, and raw spin lock.
• Add document/book.toml so mdbook build document works with the current document layout.
• Keep historical/unimplemented sync pages out of the main navigation.

Notes

The docs intentionally reference SanktaOS-style concise pages, but do not copy its minimal navigation model. Comix keeps useful fine-grained navigation in SUMMARY.md.

Verification

• mdbook build document
• cargo doc --no-deps --target riscv64gc-unknown-none-elf
• git diff --check -- document os/src
• checked SUMMARY.md links
• checked stale source path references

[gemini-code-assist]

Code Review

This pull request refactors and updates the design documentation across various subsystems of the Comix kernel, including memory management, file systems, IPC, networking, synchronization, and logging, to align them with the current codebase and clarify design boundaries. A few minor typos were found in the ext4 documentation where 'adapters.rs' was misspelled as 'adpaters.rs'.

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[gemini-code-assist]

Typo: adpaters.rs should be adapters.rs.

- `adapters.rs`: `BlockDriver` 到 ext4_rs block interface 的适配。

[gemini-code-assist]

Typo: adpaters.rs should be adapters.rs.

- `os/src/fs/ext4/adapters.rs`: 块设备适配层。