Clarify the purpose of `dy scan` command in documentation

[StoneDot]

Current Situation

The README currently states:

dy scan command executes Scan API internally to retrieve all items in the table.

This description suggests that dy scan is intended to retrieve all items, which may cause confusion about the difference between dy scan and dy import.

Proposed Change

The documentation should clarify that:

• dy scan is primarily for inspecting/sampling table items (to see what typical items look like)
• dy import is for retrieving all items from a table

Suggested Documentation Update

`dy scan` command executes Scan API internally with a default limit. 
To retrieve all items from a table, use `dy import` command.

Context

This came up in the discussion of #278 and #279, where we're considering the design of pagination in dy scan. Clarifying the intended purpose of each command will help guide these design decisions.