Add link to Zulip

[kazk]

Link to Zulip when we're ready to allow normal users to join. Write some guide to educate users how to communicate effectively.

[hobovsky]

If Gitter is going to be dropped, we need to remember to update links in existing docs too. One place I can think of is page Troubleshooting your solution, but maybe you know more.

[kazk]

Links to Gitter exists in:

• https://github.com/codewars/docs/blob/master/wiki/troubleshooting-your-solution.md
• https://github.com/codewars/docs/blob/master/wiki/community-code-of-conduct.md
• https://github.com/codewars/codewars.com/blob/master/README.md
• https://github.com/codewars/codewars.com/wiki

[kazk]

I changed my mind about opening up Zulip. I'd like to keep it private to avoid the noise, so I can keep the tab open and have the notification enabled for the mods.

Considering Dicord for the community instead.