Made it clearer that building is for contributors only in `COMPILING.md` Fixed 2 dead links in `CONTRIBUTING.md`, that were caused by separating `COMPILING.md` and file structure changed to `pr-guide.md`
* Addition of basic contributor docs. - Main contributor guide landing page. - C# codestyle doc. - Pull request guide doc. All files and structure heavily inspired by the dotnet/runtime docs: https://github.com/dotnet/runtime/tree/main/docs * fix typos and review changes * Update XML doc requirement & conversation review.