Migrate to GreptimeDB from other platforms

[nicecui]

• [ ] https://github.com/GreptimeTeam/docs/issues/1451
• [ ] Explain more when import Data from MySQL and PostgreSQL
• [ ] ClickHouse: https://github.com/GreptimeTeam/docs/issues/1445
• [ ] elasticsearch: https://github.com/GreptimeTeam/docs/issues/1446
• [ ] loki: https://github.com/GreptimeTeam/docs/issues/1447
• [ ] VictoriaMetrics: https://github.com/GreptimeTeam/docs/issues/1448
• [ ] TimescaleDB: https://github.com/GreptimeTeam/docs/issues/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

[nicecui]

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