Integrate lgtmd docs site and pin SDK to 0.3.0

[softmarshmallow]

Summary

Connect N3's existing executable guides to lgtmd dev through a root site configuration and just docs flow. The adapter replays the real application once, hands off the overview and all 28 feature guides with their captured media, and preserves the canonical guide and Reader/Contributor boundaries.

Pin the SDK to the exact crates.io release lgtmd = "=0.3.0", with its registry checksum in Cargo.lock. Update contributor setup and architecture documentation for the site integration and registry dependency.

Validation

• Native just verify: formatting, Clippy, tooling checks, 957 Rust tests passed (one existing ignored test), measurement tests, and exact guide replay.
• Full-site replay test verifies guide parity and contributor-note filtering.
• The pre-push hook reruns native verification on the clean committed branch.

Known CLI limitation

The SDK compiles and verifies successfully, but the separately distributed, unmodified lgtmd@0.3.0 CLI rejects N3's full retained media candidate with Invalid or oversized candidate file. Full-site local serving currently requires the separately verified large-media host patch in a local CLI build, as documented. This PR does not vendor that SDK/CLI patch or reduce capture quality.