docs: make getting started assistant-first

[Jammy2211]

Summary

• Put the PyAutoGalaxy AI Assistant first in README and Read the Docs getting-started guidance.
• Present conversation agents and coding agents as two ways to use one assistant.
• Introduce the remaining resources as human-readable documentation and examples while preserving existing Colab and workspace links.

API Changes

None — internal changes only. Documentation wording only; the public Python API is unchanged. See full details below.

Test Plan

• python -m pytest test_autogalaxy/ -x
• python -m sphinx --keep-going -b html docs <temporary-output-directory>
• Confirmed the scoped files contain none of the retired chat/agentic/Three Ways terminology
• Confirmed https://github.com/PyAutoLabs/autogalaxy_assistant is the intended canonical URL; its temporary 404 was explicitly accepted by the maintainer in PyAutoLens issue #645 pending creation of the assistant later today

Full API Changes (for automation & release notes)

Removed

• None.

Added

• None.

Renamed

• None.

Changed Signature

• None.

Changed Behaviour

• None.

Migration

• None required.

Generated by the PyAutoLabs agent workflow.

[Jammy2211]

Workspace PR: PyAutoLabs/autogalaxy_workspace#155