Add doc for CreateSweepableEstimator, Parameter and SearchSpace

[LittleLittleCloud]

We are excited to review your PR.

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• There's a descriptive title that will make sense to other developers some time from now.
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• Your change description explains what the change does, why you chose your approach, and anything else that reviewers should know.
• You have included any necessary tests in the same PR.

#6589

[luisquintanilla]

@LittleLittleCloud sorry for the delay. I think it looks great. I added a few comments.

[luisquintanilla]

Thanks @LittleLittleCloud. LGTM.

[codecov]

Codecov Report

Merging #6611 (640ded5) into main (6002aa8) will decrease coverage by 0.05%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6611      +/-   ##
==========================================
- Coverage   68.39%   68.34%   -0.05%     
==========================================
  Files        1176     1176              
  Lines      248210   248210              
  Branches    25915    25915              
==========================================
- Hits       169759   169650     -109     
- Misses      71676    71794     +118     
+ Partials     6775     6766       -9     

Flag	Coverage Δ
Debug	68.34% <ø> (-0.05%)	⬇️
production	62.72% <ø> (-0.06%)	⬇️
test	88.86% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/Microsoft.ML.SearchSpace/Parameter.cs	74.05% <ø> (ø)
src/Microsoft.ML.SearchSpace/SearchSpace.cs	83.40% <ø> (ø)

... and 6 files with indirect coverage changes