Added k-nn user guide and samples.

[dblock]

Description

Added k-nn user guide and samples.

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[codecov[bot]]

Codecov Report

Merging #449 (59cbe84) into main (58217d9) will increase coverage by 0.06%. The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #449      +/-   ##
==========================================
+ Coverage   71.42%   71.49%   +0.06%     
==========================================
  Files          81       81              
  Lines        7668     7668              
==========================================
+ Hits         5477     5482       +5     
+ Misses       2191     2186       -5     

see 1 file with indirect coverage changes

[dblock]

@vamshin ++ take a look pls?