docs: migrate HowToGalaxy URLs/prose to PyAutoLabs/HowToGalaxy repo

[Jammy2211]

Overview

Sub-task 3 of 3 of the HowToGalaxy extraction. The HowToGalaxy tutorial series now lives in its own repository at https://github.com/PyAutoLabs/HowToGalaxy (sub-task 1 extracted the content there, sub-task 2 removed it from autogalaxy_workspace). This task updates every HowToGalaxy reference in the PyAutoGalaxy docs and README to point at the new standalone repo.

Plan

• Rewrite Colab URLs across the docs/howtogalaxy/chapter_*.rst chapter landings to the new repo at tag 2026.4.13.6, dropping the redundant howtogalaxy/ path segment (new repo root is the tutorial series).
• Reword README "new starter" links and the HowToGalaxy callout block to frame HowToGalaxy as a standalone repo alongside autogalaxy_workspace.
• Update docs/general/workspace.rst, docs/howtogalaxy/howtogalaxy.rst, docs/index.rst, docs/overview/overview_2_new_user_guide.rst, and paper/paper.md with the same framing.
• Update the scripts/howtogalaxy/ mention in CLAUDE.md — it's now an external repo.
• Verify no stale autogalaxy_workspace/.../howtogalaxy references remain.

Detailed implementation plan

Affected Repositories

• PyAutoGalaxy (primary)

Work Classification

Library (docs)

Branch Survey

Repository	Current Branch	Dirty?
PyAutoGalaxy	main	clean

Suggested branch: feature/howtogalaxy-sub3
Worktree root: ~/Code/PyAutoLabs-wt/howtogalaxy-sub3/

URL transform (colab chapter links)

BEFORE: https://colab.research.google.com/github/PyAutoLabs/autogalaxy_workspace/blob/2026.4.13.6/notebooks/howtogalaxy/chapter_X_Y/tutorial_Z.ipynb
AFTER:  https://colab.research.google.com/github/PyAutoLabs/HowToGalaxy/blob/2026.4.13.6/notebooks/chapter_X_Y/tutorial_Z.ipynb

Files to rewrite (12 files, 56 occurrences)

File	Refs
README.rst	5
docs/general/workspace.rst	3
docs/index.rst	10
docs/overview/overview_2_new_user_guide.rst	3
docs/howtogalaxy/howtogalaxy.rst	6
docs/howtogalaxy/chapter_1_introduction.rst	6
docs/howtogalaxy/chapter_2_modeling.rst	8
docs/howtogalaxy/chapter_3_search_chaining.rst	3
docs/howtogalaxy/chapter_4_pixelizations.rst	5
docs/howtogalaxy/chapter_optional.rst	3
paper/paper.md	3
CLAUDE.md	1

Follow-up (out of scope for this PR)

Register HowToGalaxy in PyAutoBuild so /pre_build can auto-tag future HowToGalaxy versions in lockstep with autogalaxy_workspace.

Related

• Umbrella issue: feat: bootstrap PyAutoLabs/HowToGalaxy from autogalaxy_workspace autogalaxy_workspace#35
• Sub-task 1 (extract): feat: bootstrap HowToGalaxy repo with tutorial content and CI HowToGalaxy#1
• Sub-task 2 (remove from workspace): refactor: remove howtogalaxy tutorial dirs (now in PyAutoLabs/HowToGalaxy) autogalaxy_workspace#37
• Template precedent: docs: update HowToLens references after extraction to standalone repo PyAutoLens#468 (HowToLens sub-task 3)