[mj-will]

Add a hidden Related Packages toctree linking to the main Sequince documentation and sequince-density. Both links appear in the persistent sidebar so readers can return to the main docs from any tutorial, matching the navigation in sequince-dev/sequince#140.

Validation:

• All pre-commit hooks passed; Ruff lint and format checks passed.
• Existing tests: 46 passed, 4 skipped.
• Built a standalone Sphinx fixture using the exact new toctree and the same theme with warnings treated as errors; confirmed both sidebar links render on a child page.
• Full documentation build was blocked by missing validated example outputs for this source revision. No sampling was performed and no output provenance checks were changed.
• git diff --check passed.

Prepared in an isolated checkout to preserve existing local work.