Remove references to "sandboxes" from docs.

[m-renaud]

Sandboxes are obsolete with the launch of Nix-style/new-/v2 commands, and thus
should not be given high visibility in the docs for the current version. If docs
are needed then the docs for a previous version of cabal can be used.

[ci skip]

This partially addresses #6194.

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• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
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[phadej]

please merge as soon as CI turns green.