Add example for a GitHub code host integration
Created by: JeanPaulSG
This change adds a simple example of adding a public repo in Site Admin. Also corrected some interface names (manage repositories -> Manage code hosts). In part this change addresses feedback in https://sourcegraph.slack.com/archives/C0W2E592M/p1652920840870179 and it addresses, in part, the issue that is temporarily housed here: https://github.com/orgs/sourcegraph/projects/241/views/2
Test plan
Language edit by PE team member. @blvckcoffee @ryanslade is suggested reviewer