Reorder trainer flags to alphabetical order

[SkafteNicki]

What does this PR do?

Fixes #<issue_number>

Reorders the main documentation on trainer flags to be in alphabetical order (which it nearly is, so I assume that is the intention):
https://lightning.ai/docs/pytorch/stable/common/trainer.html#trainer-flags

Note that a lot of trainer flags are actually undocumented in this list. Going to add them in a follow up PR.

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📚 Documentation preview 📚: https://pytorch-lightning--21203.org.readthedocs.build/en/21203/