Update Plugins doc

[kaushikb11]

What does this PR do?

Update Plugins doc with the latest v1.6 changes
also fixes: #12492 and replace training type instances with strategy.

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

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cc @Borda @rohitgr7 @justusschock @kaushikb11 @awaelchli @akihironitta @ananthsub

[akihironitta]

LGTM! 🚀

Should we update these occurrences of "training type plugin", too?

$ grep -rnIi "training type" docs/source
...
docs/source/common/trainer.rst:1451:Supports passing different training strategies with aliases (ddp, ddp_spawn, etc) as well as custom training type plugins.
docs/source/common/trainer.rst:1461:.. note:: Additionally, you can pass your custom training type plugins to the ``strategy`` argument.

[tchaton]

LGTM !