Legacy Channel Entries API docs needs to be removed

[derekjones]

Description of the problem

The legacy Channel Entries API was deprecated in v3, and is not viable for v4+ due to the improved storage architecture. But those pages are still available even in the v5 docs, with not mention to not use them.

Suggested change: add a deprecation notice to the v3 docs for this API since that's the version it was deprecated in, and remove those pages entirely from the v4 and v5 docs.

See: https://github.com/ExpressionEngine/ExpressionEngine/issues/80