Update documentation

[martincostello]

• Add a table of providers with NuGet, MyGet and documentation links for each provider.
• Update the list of active maintainers.
• Update example code snippet for ASP.NET Core 2.2.
• Add graphs for builds as well as homogenise badges.
• Update license links to be HTTPS.
• Link to the samples directory where we can put a child README instead of directly to the sample app's directory.
• Add solution items to the Visual Studio solution.
• Add an .editorconfig file.
• Run code clean-up.
• Add a contributing guide
• Add a Code of Conduct
• Add a Pull Request template
• Add issue templates