Migrate to GreptimeDB from other platforms

[nicecui]

• Write the overview document for the migration documents #1451
• Explain more when import Data from MySQL and PostgreSQL
• ClickHouse: Migrate from ClickHouse #1445
• elasticsearch: Migrate from Elasticsearch #1446
• loki: Migrate from Loki #1447
• VictoriaMetrics: Migrate from VictoriaMetrics #1448
• TimescaleDB: Migrate from TimescaleDB #1449

[nicecui]

The outline of the migration documents should have:

# Migrate from XXX

## Data Model in Difference

## Database Connection Information

## Ingest Data to both GreptimeDB and XXX simultaneously

## Export Data from XXX

## Import Data into GreptimeDB