Implement service neutral documentation

- [x] Setting up your development environment
- [x] Setting up Rust on Cloud Shell
- [x] Bootstrap the user guide
- [x] How to initialize a client and make one RPC
- [x] #1052
- [ ] User-guide section on how to modify resources with a OCC loop
- [ ] User-guide section on how to call functions that create resources
- [x] #1078
- [ ] User-guide section on how to set a timeout for just one RPC
- [ ] User-guide section on how to set a timeout for a client
- [ ] User-guide section on how to change the backoff policy for a client
- [ ] User-guide section on how to change the backoff policy for just one RPC
- [ ] User-guide section on how to change the retry throttler for a client
- [ ] User-guide section on how to share the retry throttler across two clients
- [x] #1120
- [x] #1121
- [ ] User-guide section on how to create a custom retry policy
- [ ] User-guide section on how to create a custom backoff policy
- [ ] User-guide section on how to create a custom polling error policy
- [ ] User-guide section on how to create a custom retry throttler