Cloud identity docs

This PR adds the Cloud Identity service to our documentation. We need the following language samples:

- [x] Bash
- [ ] C#
- [x] Golang
- [ ] Java
- [ ] Javascript
- [x] PHP
- [ ] Python
- [x] Ruby
The use of `{username}` here may be confusing since the same placeholder appears for authentication examples but is not necessarily the same user being referred to here. This is especially obvious in the C# examples because the line which creates the "identity object" is included in this sample, and it uses the other meaning of `{username}`.