Add xmldoc comments to code

[benjiro]

Issue: #20

• Document public code using xmldoc, and generate doc file
• Avoid parallel test running to fix race condition when setting multiple provides on the Singleton instance which causes problems in the OpenFeature singleton tests

[codecov-commenter]

Codecov Report

Merging #21 (b71ab26) into main (05de4b5) will increase coverage by 2.82%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #21      +/-   ##
==========================================
+ Coverage   96.50%   99.32%   +2.82%     
==========================================
  Files          15       15              
  Lines         286      297      +11     
  Branches       22       22              
==========================================
+ Hits          276      295      +19     
+ Misses          7        0       -7     
+ Partials        3        2       -1     

Impacted Files	Coverage Δ
src/OpenFeature/Constant/Reason.cs	100.00% <ø> (ø)
src/OpenFeature/Extension/EnumExtensions.cs	100.00% <ø> (+20.00%)	⬆️
...enFeature/Extension/ResolutionDetailsExtensions.cs	100.00% <ø> (ø)
src/OpenFeature/Hook.cs	100.00% <ø> (ø)
src/OpenFeature/Model/ClientMetadata.cs	100.00% <ø> (ø)
src/OpenFeature/Model/FlagEvaluationOptions.cs	100.00% <ø> (ø)
src/OpenFeature/Model/HookContext.cs	100.00% <ø> (ø)
src/OpenFeature/Model/Metadata.cs	100.00% <ø> (ø)
src/OpenFeature/NoOpProvider.cs	100.00% <ø> (ø)
src/OpenFeature/Error/FeatureProviderException.cs	100.00% <100.00%> (ø)
... and 8 more

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[oleg-nenashev]

LGTM after a quick check