Optimized example configuration to enhance readability

[chunshao90]

Describe This Problem

type in analytic.storage, analytic.storage.local_store, analytic.storage.remote_store is confusing. https://github.com/CeresDB/ceresdb/blob/8cccedd01b2ac33768a6678f133e51a394468bda/docs/example.toml#L12

[analytic.storage]
type = "Cache"
cache_opts = { max_cache_size = 10000000 }

[analytic.storage.local_store]
type = "Local"
data_path = "/tmp/ceresdb"

[analytic.storage.remote_store]
type = "Aliyun"
key_id = "key_id"
key_secret = "key_secret"
endpoint = "endpoint"
bucket = "bucket"

Proposal

Add some notes to explain what the type means.

Additional Context

TODO: Add configuration explanation in user guide.