all: add performance centered docs for data intensive clients

As I was helping someone with a question the other day and was looking through some of our documentation it became apparent to me that we could do a better job having `Performance Tuning` section in some of our veneer client libs. Pretty much all of the veneers are our data intensive clients and depending on the type of workload users should likely change some parameters.

Examples(some things we might already doc on but for ideas sake)
- When to set the chunk size and to what in GCS
- Batching logs in Logging
- Using managedwriter in BQ
- When to ack a message in pubsub
- gRPC connection pool size
- Large transaction in Spanner
- Batch mutations in datastore
- etc