sql/create_index.md: new page

Изменения

Address changes from picodata!929 (merged)

Версия: Picodata 24.3.0-65-g6043629f

Refer:

• https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/create_index/#box-space-create-index
• https://www.tarantool.io/en/doc/latest/concepts/data_model/indexes/

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• Close #226 (closed)
• Changes should be cherry-picked to 24.2: no
• Staging: https://docs.binary.picodata.io/picodata/asabirov_add_create_index_md/reference/sql/create_index
• See also sbroad#580 (closed)