Revamped outlier detection documentation

[ojcobb]

So far just a first draft of overview page. Sharing early to gather feedback before working towards something more polished.

Methods pages for individual detectors and updated doctrings will then follow.

[codecov]

Codecov Report

Merging #834 (1db8ce2) into master (8b2dcbb) will decrease coverage by 0.07%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #834      +/-   ##
==========================================
- Coverage   81.86%   81.79%   -0.07%     
==========================================
  Files         159      159              
  Lines       10317    10317              
==========================================
- Hits         8446     8439       -7     
- Misses       1871     1878       +7     

see 1 file with indirect coverage changes

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