Added GitHub-Repo to Useful links (#1661)

* Added GitHub-Repo to Useful links

I really missed that link ;)
additionally: the link for the slack channel doesn't work anymore. it says it is expired.

* changed slack-channel-link

Author: janschreier

docfx_project/index.md

@@ -21,6 +21,7 @@ Puppeteer Sharp is a .NET port of the official [Node.JS Puppeteer API](https://g
 
 # Useful links
 
-* Slack channel [#puppeteer-sharp](https://puppeteer.slack.com/join/shared_invite/enQtMzU4MjIyMDA5NTM4LWI0YTE0MjM0NWQzYmE2MTRmNjM1ZTBkN2MxNmJmNTIwNTJjMmFhOWFjMGExMDViYjk2YjU2ZmYzMmE1NmExYzc)
+* [GitHub-Repo](https://github.com/hardkoded/puppeteer-sharp)
+* Slack channel [#puppeteer-sharp](https://www.hardkoded.com/goto/pptr-slack)
 * [StackOverflow](https://stackoverflow.com/search?q=puppeteer-sharp)
-* [Issues](https://github.com/kblok/puppeteer-sharp/issues?utf8=%E2%9C%93&q=is%3Aissue)
+* [Issues](https://github.com/kblok/puppeteer-sharp/issues?utf8=%E2%9C%93&q=is%3Aissue)