[doc] Add Fault Tolerant Documentation Page

[tchaton]

What does this PR do?

This PR adds documentation for Fault Tolerant Training.

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

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[codecov]

Codecov Report

Merging #9256 (ad8c571) into master (eacece1) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #9256   +/-   ##
======================================
- Coverage      92%     92%   -0%     
======================================
  Files         176     176           
  Lines       14810   14819    +9     
======================================
+ Hits        13662   13666    +4     
- Misses       1148    1153    +5     

[SeanNaren]

Extremely excited, great work @awaelchli @tchaton and others who helped in the effort to add this to Lightning :)

[ethanwharris]

LGTM 😃