Update docs for multiple optimizers in 2.0

[awaelchli]

What does this PR do?

For a summary of changes to how multiple optimizers are handled in the new version, see #16539.
This PR brings the necessary documentation changes.

Before submitting

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• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
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• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 pytorch_lightning: Docs

Check ID	Status
make-doctest (pytorch)	success	✅
make-html (pytorch)	success	✅

These checks are required after the changes to docs/source-pytorch/common/lightning_module.rst, docs/source-pytorch/common/optimization.rst, docs/source-pytorch/guides/speed.rst, docs/source-pytorch/model/build_model_advanced.rst, docs/source-pytorch/model/manual_optimization.rst, docs/source-pytorch/starter/introduction.rst.

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Thank you for your contribution! 💜

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