Adding minimal demos for various client api to write data into Greptime database about demo-scene

[tisonkun]

We currently add demos covering /v1/influxdb/write and /v1/influxdb/api/v2/write.

In the following demos we'd covering Prometheus usages, we will cover /v1/prometheus/xxx.

SQL and PromQL query is included in every demo when we are analyzing the data. I suggest we can add some real-world scenarios to guide users how to write queries and how to migrate from other query slang like InfluxQL. We have a good start at https://docs.greptime.com/user-guide/migrate-to-greptimedb/migrate-from-influxdb#query-data. May or may not it should be part of our demos.

I don't have ideas about /v1/oltp/xxx yet.