docs: add index explanations in sql `desc`

add explanations in [DESC_STATEMENT](https://github.com/4paradigm/OpenMLDB/blob/59fbca2af07bd970edc855f3c1dd0f21268ff17c/docs/zh/reference/sql/ddl/DESC_STATEMENT.md)

add a ref in quickstart

The issue points to a specific documentation file (DESC_STATEMENT.md) and mentions adding explanations for indexes in SQL DESC. Start by locating the file in the docs/zh/reference/sql/ddl/ directory. Examine the existing content to understand the DESC statement format. Then, research how indexes are described in OpenMLDB's SQL syntax, possibly by checking the codebase or other documentation. The goal is to add clear explanations about indexes to that file and update the quickstart reference.

Written by the indexing model from the issue text.