modify tensor_introduction doc (#4297)

* modify tensor_introduction doc

* modify according to nielang's advise, change picture quote method

* fix bugs

* fix some md format bugs

* Update tensor images links

* fix images link bugs

* fix image link bugs

* fix image link bugs

* fix image link bugs

* fix image link bugs

* point Tensor's necessity; add the same and difference between tensor and numpy array

* Modify some descriptions.

* Update tensor_introduction_cn.md

* Update tensor_introduction_cn.md

* Update tensor_introduction_cn.md

* Update tensor_introduction_cn.md

* Update tensor_introduction_cn.md

Co-authored-by: moguguo <[email protected]>

Author: betterpig