docs: remove classic packaging page

[Margarita215729]

Closes #834

• removes the retired classic-packaging guide from the source and navigation
• updates the guides overview to point readers to the current packaging guides
• preserves the existing /guides/packaging-classic/ URL with a redirect to /guides/packaging-simple/, following the existing redirect-stub pattern

Checks:

• npm exec --yes --package=mystmd@1.10.1 -- myst build --html
• uvx pre-commit run --files .readthedocs.yaml docs/guides/index.md docs/myst.yml redirect-stubs/guides/packaging-classic/index.html
• verified the generated simple-packaging page and copied redirect target

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📚 Documentation preview 📚: https://scientific-python-cookie--855.org.readthedocs.build/

[henryiii]

Seems fine to me. I'll give it a bit more time for any last objections then merge.