Add summary NuGet badges

[martincostello]

Added NuGet package badges for the latest version and the overall number of downloads.

[Copilot]

Pull Request Overview

This PR adds NuGet package badges to the README.md file to display the latest version and total download count for the Polly package.

• Added two NuGet badges displaying package version and download statistics
• Positioned badges at the top of the README for visibility
• Added corresponding badge reference URLs at the bottom of the file

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.12%. Comparing base (6d5fcd6) to head (ab99027).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2724   +/-   ##
=======================================
  Coverage   96.12%   96.12%           
=======================================
  Files         309      309           
  Lines        7118     7118           
  Branches     1008     1008           
=======================================
  Hits         6842     6842           
  Misses        222      222           
  Partials       54       54           

Flag	Coverage Δ
linux	96.12% <ø> (ø)
macos	96.12% <ø> (ø)
windows	96.11% <ø> (ø)

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