Add warning about imports to all.py

[nbruin]

Warn about not importing from all and instead use import_statements to figure out where to import from instead.

Fixes #41213

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[user202729]

We have .. WARNING::, but this part doesn't get rendered to HTML page anyway.

caveat: it isn't entirely clear this is what this module is meant to be. That said, it is true that the general direction is for imports in the Sage library to not import from this module.

[nbruin]

it isn't entirely clear this is what this module is meant to be.

It's the global namespace for the interactive prompt. Hence the definition of sage_globals in there that captures the global namespace of that module. It has some other configuration of the interactive state as well.