[docs] Clear up default logging, showing you don't need to pass a logger

[SeanNaren]

What does this PR do?

Clear up that you do not need to provide a logger to the Trainer, as by default it supports tensorboard.

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[awaelchli]

Nice!
careful not to commit the _notebooks changes :)

[SeanNaren]

As @awaelchli brought up, this does duplicate information in some of the start guide, however when I googled lightning logging this was the first page that came up!

[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #9408 (411c3d5) into master (439db22) will increase coverage by 1%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #9408    +/-   ##
=======================================
+ Coverage      92%     93%    +1%     
=======================================
  Files         179     180     +1     
  Lines       14904   14964    +60     
=======================================
+ Hits        13750   13903   +153     
+ Misses       1154    1061    -93