[Doc] Add Doc of Detection Transformers

[Li-Qingyun]

Motivation

We (me, @jshilong, @LYMDLUT, and @KeiChiTse) have refactored DETR-like models, to enhance the usability and readability of our codebase. This PR add a tutorial doc about the new codebase.

[Li-Qingyun]

Please refer to the compatiblility doc to update this doc. we should list more details like batch first, rename issues,etc.

Okkkk, We are making an outline.

[Li-Qingyun]

For convenience, I temporarily put the Chinese document together with the English document and images, and will adjust the file location after most of the writing is done. （I wrote Chinese first, then translated it to English. Hence, CN doc has been added.）

[ZwwWayne]

Invite @ZCMax to review

[jshilong]

@Li-Qingyun This PR can be merged after adding the compatibility part

[Li-Qingyun]

I'll try to rebase the branch and then resolve the conflicts.