Fix broken links for antlr download/csharp target.

I am using Antlr for the first time and it is a bit of frustrating experience just finding basic starting info.

For instance I start out here - this is the top result when you google 'antlr4 getting started': https://github.com/tunnelvisionlabs/antlr4/blob/master/doc/getting-started.md - scroll down to the Windows section, click the link https://www.antlr.org/download/ and get 404 page not found.

Then I am trying navigate the csharp version so I ended up here: https://github.com/antlr/antlr4/blob/master/doc/csharp-target.md. Scroll down How do I get started, click the link for (Git repo page for ANTLR C# runtime) https://github.com/antlr/antlr4/tree/master/runtime/CSharp, and then I am sitting in some sort of folder? Where are the Instructions??

After hunting around a bit I think it's supposed to be this? https://github.com/antlr/antlr4/blob/master/runtime/CSharp/src/README.md.

A bit disorienting for a brand new first time user. Thanks for understanding!!

Read doc/getting-started.md and doc/csharp-target.md, then inspect runtime/CSharp/src/README.md as the likely destination for C# runtime instructions. Verify the download and repository links, update broken or confusing references, and confirm that a new user can reach the download page and setup instructions from both documents.