[Docs] Add diagram about hedging's context and callbacks

[peter-csala]

Pull Request

The issue or feature being addressed

• [Docs] Hedging context #1749

Details on the issue fix or feature implementation

• As it was discussed in the review comments: it makes sense to visualize the sequence of actions related to context and callbacks
• @martintmk Can you please double check whether the diagram depicts everything and in the right order?
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Confirm the following

• I started this PR by branching from the head of the default branch
• I have targeted the PR to merge into the default branch
• I have included unit tests for the issue/feature
• I have successfully run a local build

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (41d4e8d) 84.56% compared to head (fb30254) 84.56%.
Report is 3 commits behind head on main.

Additional details and impacted files

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##             main    #1751   +/-   ##
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  Coverage   84.56%   84.56%           
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  Files         307      307           
  Lines        6790     6790           
  Branches     1043     1043           
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  Hits         5742     5742           
  Misses        839      839           
  Partials      209      209           

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linux	84.56% <ø> (ø)
macos	84.56% <ø> (ø)
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[martintmk]

Some real-case scenario where this is used:

https://github.com/dotnet/extensions/blob/3038d565967654ffb92cdd5edf434930c9249e91/src/Libraries/Microsoft.Extensions.Http.Resilience/Hedging/ResilienceHttpClientBuilderExtensions.Hedging.cs#L84