Publish usage guides on Read the Docs; merge docs into doc/

[trailcode]

Summary

• Publish user guides on Read the Docs (Sphinx + MyST from root usage*.md / scripting.md)
• Consolidate Sphinx build files: docs/ → doc/ (screenshots remain in doc/gen/)
• Add ezycad_doc_style.md for documentation conventions; extend usage guides (Sketch List, image underlay, RTD links)
• Add splash image to README.md and usage.md; CI doc build via GitHub Actions

Closes #106

Changes

Area	Details
RTD / CI	.readthedocs.yaml, .github/workflows/docs.yml, doc/conf.py (sync guides, icons, splash, doc/gen/; asset checks)
Layout	doc/index.rst, doc/requirements.txt, doc/readthedocs.md; remove docs/
Guides	usage.md, usage-sketch.md, usage-settings.md updates; README.md splash + style links
Style	New ezycad_doc_style.md; ezycad_code_style.md points to doc guide

Test plan

• GitHub Actions Documentation workflow passes
• Read the Docs build green after merge (fail_on_warning: true)
• Local: pip install -r doc/requirements.txt then sphinx-build -b html -W doc doc/_build
• Spot-check https://ezycad.readthedocs.io/en/latest/usage.html — icons, screenshots, splash
• Help → Usage Guide opens RTD (if app changes are on main separately, skip)

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