docs: Remove dead link to citizencodeofconduct.org.
docs: Remove dead link to citizencodeofconduct.org.

This website has been defunct for years, and seems unlikely to return.

I have removed the dead link to citizencodeofconduct.org that
consistently breaks the `tools/test-documentation` test tool.

kou/zulip,eeshangarg/zulip,hackerkid/zulip,kou/zulip,punchagan/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip,punchagan/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,andersk/zulip,zulip/zulip,rht/zulip,kou/zulip,hackerkid/zulip,zulip/zulip,zulip/zulip,zulip/zulip,punchagan/zulip,punchagan/zulip,eeshangarg/zulip,punchagan/zulip,hackerkid/zulip,kou/zulip,andersk/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,hackerkid/zulip,hackerkid/zulip,kou/zulip,punchagan/zulip,kou/zulip,hackerkid/zulip,zulip/zulip,andersk/zulip,rht/zulip,punchagan/zulip