Implement service neutral documentation

[mpeddada1]

• [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
• [ ] #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