Add Promoted CLI to API Reference Section

[speediedan]

What does this PR do?

Fixes #14071

Thanks for your immensely valuable contributions to the ML research community!

I've noticed that since the recent promotion of the cli out of utilities, the new location is missing its reference in the API Reference:

python -msphinx.ext.intersphinx https://pytorch-lightning.readthedocs.io/en/latest/objects.inv | grep "pytorch_lightning.*.cli.LightningCLI "

pytorch_lightning.utilities.cli.LightningCLI api/pytorch_lightning.utilities.cli.html#pytorch_lightning.utilities.cli.LightningCLI

This PR fixes the issue by simply adding the new api location (in addition to keeping the deprecated existing one) so that intersphinx references to the CLI can be updated to the new location and that the new location is properly reflected in the API Reference section:

python -msphinx.ext.intersphinx ../build/html/objects.inv | grep 'pytorch_lightning.*.cli.LightningCLI '

pytorch_lightning.cli.LightningCLI       api/pytorch_lightning.cli.LightningCLI.html#pytorch_lightning.cli.LightningCLI
pytorch_lightning.utilities.cli.LightningCLI api/pytorch_lightning.utilities.cli.html#pytorch_lightning.utilities.cli.LightningCLI
...

Thanks again!

Does your PR introduce any breaking changes? If yes, please list them.

None

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