docs: make the architecture diagram readable and repair a page heading

[imran-siddique]

The deployed architecture diagram shrinks its labels on mobile. Wrap it in a labeled, keyboard-focusable horizontal scroll region using the shared diagram styles. Also keep the #461 issue reference inside its hardware-validation paragraph, where it previously became a second H1.

Validation: strict MkDocs build and CI docs-check pass. The current full CI run fails 20 runtime tests around Agent Manifest verification and a TPM error string; it is not ready to merge. On clean main 6e5d0ce with agent-manifest 0.12.0, the targeted Manifest/TPM tests reproduce 9 failures and 10 passes. This branch changes only two documentation pages.

[imran-siddique]

@agentrust-io/maintainers please review this PR and submit an approving review if the changes look correct. All checks are green; maintainer approval is still required to merge.

This keeps the architecture diagram readable on small screens with a labeled, keyboard-focusable horizontal scroll region. The shared contrast styles have merged, and the local preview was checked in both themes.

[imran-siddique]

CI blocker reproduced on clean main 6e5d0ce: tests/unit/test_agent_manifest.py and tests/unit/test_tpm_chained_verify.py give 19 passed with agent-manifest 0.11.2, versus 9 failed and 10 passed with 0.12.0. The latter failures match the Manifest MISMATCH and TPM message failures in this PR run. The branch changes only two documentation pages and docs-check passes. Please keep this unmerged until compatibility with the new SDK is resolved.