docs: clarify assistant and documentation paths

[Jammy2211]

Summary

• separate the PyAutoLens AI Assistant and human-readable documentation into clearly headed onboarding routes
• put the AI Assistant first on the Read the Docs Start Here page
• split assistant guidance from the conventional New User Guide content

API Changes

None — documentation-only changes.

Test Plan

• git diff --check
• python -m sphinx -b html docs /tmp/assistant_docs_hierarchy_pyautolens
• python -m pytest test_autolens/ -x
• Confirm generated HTML contains the new headings in the intended order

Generated by the PyAutoLabs agent workflow.

[Jammy2211]

Workspace PR: PyAutoLabs/autolens_workspace#330