Clarify dynamic mapping behavior in Elasticsearch quick start

[Copilot]

The Elasticsearch quick start page describes dynamic: true in a way that can be read as if mappings do not change when new fields are indexed. This update makes the behavior explicit: new fields cause the mapping to update dynamically.

• Problem

  • The Combine dynamic and explicit mappings section used wording that blurred the distinction between manually updating a mapping and Elasticsearch dynamically adding fields to it.

• Change

  • Reworded the affected sentence in solutions/search/get-started/index-basics.md to state that, when dynamic is true, the mapping updates dynamically as new fields are added to documents.
  • Kept the surrounding explanation and page structure unchanged.

• Updated wording

  When an index has the `dynamic` flag set to `true`, the mapping updates dynamically when you add new fields to documents, which allows you to combine explicit and dynamic mappings.

• Fixes [Website]: 关于官方文档中Quick starts中Combine dynamic and explicit mappings存在误导性说明的问题 #6849