docs: fold the docs-site SDK appendix into docs/

[Vertax42]

Why

The XTac-UMI G1 documentation site is being trimmed. Its three SDK pages (概览 / 安装与构建 / 示例) are retired and the site now just links to this repo's docs/. This PR merges into the existing files everything those pages said that docs/ did not already cover, or covered with older wording. Nothing that docs/ already has is duplicated.

What changed

File	Added
ARCHITECTURE.md (+33)	§1.1 components by role and command-set version; §8 note that the lerobot layer does not use the SDK Camera
INSTALL.md (+48/−4)	Python ≥ 3.10 per pyproject, Ubuntu 22.04/24.04, cwd note for submodule use, --no-build-isolation to match README, new §5c add_subdirectory integration (no find_package), §6 hardware self-check
EXAMPLES.md (+62/−1)	ota_update.py CRC32/role checks and --force; C++ leader_demo smoke program with a minimal C++17 example; follower motor-control section using the scripts that currently exist
CALIBRATION.md (+6/−2)	once-per-leader note; same-side duplicate SN refusal
USAGE.md (+8/−6)	§0 healthy-output and troubleshooting rows for empty SN / Unknown role and ModemManager

Every added statement was checked against the current sources (pyproject.toml, python/examples/, cpp/include). Site content that is now stale was deliberately not carried over: callbacks-on-producer-thread, the deleted rerun_dual_with_tracker / v4l2_* / motor_mit_control scripts, and the "falls back to dividing by a constant" calibration claim (README says construction raises ProtocolError).

Two things to confirm

• INSTALL.md §5a now says pip install -e . --no-build-isolation, matching README. If the omission in INSTALL was intentional, revert that one line.
• New EXAMPLES.md sections are in Chinese, following the file's most recent sections; the ota_update.py table row stays English.

Noticed but not changed (outside this PR)

USAGE.md §1.4 and README still use --side right although CHANGELOG Unreleased switched to a positional argument; ARCHITECTURE says command set V2.1 while README says V2.2; the ARCHITECTURE module diagram still lists "MIT control, V4L2 probes" under python/examples/.

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