User guide for table sharding

[killme2008]

We currently offer a table-sharding contributor guide at https://docs.greptime.com/contributor-guide/frontend/table-sharding#table-sharding. However, a user guide for table-sharding is absent. It would be beneficial to provide a table-sharding user guide for cluster users, covering:

• Explanation of greptimedb table-sharding functionality
• Instructions on table partitioning in GreptimeDB with examples
• Guidelines for managing table shardings

[killme2008]

@waynexia Assign to you, but maybe @fengjiachun or @WenyXu can help

[nicecui]

Maybe we can add the partition guides to this cluster document.