Update strategy flag in docs

[kaushikb11]

What does this PR do?

Update strategy flag in the docs. It will replace accelerator and plugins flags for training strategies.

Related Issues: #9053, #9932

The follow-up PR would be updating the Trainer flag documentation (accelerator, strategy) and required changes for ddp_cpu

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

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[rohitgr7]

might need to update here too: https://github.com/PyTorchLightning/pytorch-lightning/blob/8f14e77d76b45638626850d9170884e53da99a9f/pytorch_lightning/trainer/trainer.py#L190-L191

[codecov]

Codecov Report

Merging #10000 (bfed77f) into master (2ed92ec) will decrease coverage by 0%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #10000   +/-   ##
=======================================
- Coverage      93%      93%   -0%     
=======================================
  Files         180      180           
  Lines       15892    15892           
=======================================
- Hits        14722    14720    -2     
- Misses       1170     1172    +2     

[awaelchli]

find and replace 📎 ✏️