Docs updates

[Rob-Hague]

• Use /// <value> instead of /// <returns> for properties (the latter is only meant for methods as it turns out - still works in VS but not in docfx)
• misc doc updates in SshCommand
• Update README - remove some fluff at the start and move the examples upfront. Base it somewhat on the template of MS packages as suggested in ssh.net is missing NuGet package README file #1311 e.g. https://www.nuget.org/packages/System.Text.Json