docs: remove howtofit/ tree and point tutorials to standalone repo

[Jammy2211]

Summary

Sub-task 3 of the HowToFit extraction (umbrella issue: PyAutoLabs/autofit_workspace#38). Sub-1 created the standalone PyAutoLabs/HowToFit repo; sub-2 removed the howtofit/ tree from autofit_workspace. This PR does the PyAutoFit library side: deletes docs/howtofit/ from the Sphinx build, removes the toctree entries that referenced those chapters, and rewrites every lingering pyautofit.readthedocs.io/howtofit/… URL and autofit_workspace/…/howtofit/… URL to point at the standalone HowToFit repository's notebooks.

API Changes

None — documentation and prose only. No Python source, tests, or config files touched.
See full details below.

Test Plan

• pytest test_autofit/ -x passes (no library code changed, should be clean)
• Verify no howtofit URL references remain: grep -n "howtofit" --include='*.rst' --include='*.md' --exclude-dir=build returns zero matches (case-sensitive)
• ReadTheDocs build on this branch succeeds without broken-toctree warnings

Full API Changes (for automation & release notes)

Removed

• docs/howtofit/howtofit.rst (Sphinx chapter landing page)
• docs/howtofit/chapter_1_introduction.rst
• docs/howtofit/chapter_2_scientific_workflow.rst
• docs/howtofit/chapter_3_graphical_models.rst
• docs/index.rst :caption: Tutorials: toctree block (empty after chapter deletions)

Changed (URL rewrites)

• docs/api/{analysis,model,plot,priors,samples,searches,database}.rst — "HowToFit: introduction chapter" link now points to github.com/PyAutoLabs/HowToFit
• docs/general/workspace.rst — HowToFit section rewritten from "part of the workspace" framing to "standalone repo" framing
• docs/features/graphical.rst — two prose references to the HowToFit graphical-models chapter now link to PyAutoLabs/HowToFit/blob/main/notebooks/chapter_3_graphical_models
• docs/overview/statistical_methods.rst:30 — hierarchical-models tutorial URL moved to PyAutoLabs/HowToFit/blob/main/notebooks/chapter_3_graphical_models/tutorial_4_hierachical_models.ipynb
• docs/cookbooks/multiple_datasets.rst — graphical-models chapter URL repointed
• docs/science_examples/astronomy.rst — HowToFit lecture series URL repointed
• README.rst — header link + two in-body references rewritten; autofit_workspace sentence split to separate autofit_workspace and HowToFit
• docs/index.rst — two README-style HowToFit references rewritten (this file duplicates parts of README.rst)
• paper/paper.md — two JOSS-paper prose references updated to describe HowToFit as a standalone/complementary repo rather than "included in the workspace"

API Migration

Not applicable — no API surface changed.

Followups

• Register the howtofit build target in PyAutoBuild so the HowToFit notebooks regenerate automatically from scripts (tracked on umbrella issue Extract howtofit into its own repository autofit_workspace#38).

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