Include readme for dotnet/NuGet

### Describe the feature

Dotnet Packages published on NuGet currently do not include a readme.
E.g. https://www.nuget.org/packages/Amazon.CDK.Lib

`jsii-pacmak` should include a readme into the package so it shows up on NuGet.org
E.g. like https://www.nuget.org/packages/AutoMapper#readme-body-tab

### Use Case

As a NuGet user I'm always frustrated when I have to click through to GitHub to see examples or usage of the package.

`jsii-pacmak` already includes the readme for `npm`, `pypi.org` and `pkg.go.dev`

### Proposed Solution

https://docs.microsoft.com/en-us/nuget/reference/nuspec#readme

Usually a README.md will be included inside the `npm` package, so it should be possible to extract the file and include it similar to the logo.

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

jsii 1.65.0

### Environment details (OS name and version, etc.)

wsl2

Start at the jsii-pacmak NuGet packaging path and inspect how README.md is handled for npm, pypi.org, and pkg.go.dev. Compare the generated NuGet metadata with the NuGet nuspec readme requirements; done when packages such as Amazon.CDK.Lib display the README on NuGet.org.

Written by the indexing model from the issue text.